Well, you know, I own all three systems, but I'm only looking forward to 2 Wii games and they didn't make it into my Top 5, and I'm only looking forward to 1 X360 game, but it isn't in my Top 5 most anticipated games. I also anticipated some multiplatform games, but they are not among the Top5. It just has to do with personal taste in games IMO.

I don't think you're thinking logically because if people own all three consoles even if they prefer 1 over the other when it comes to multi plats they should buy the best version.  I prefer the 360 but I still bought the PS3 version of Final Fantasy 13 because it was the better version.  I rarely game on the PC because I am a console gamer first and foremost but I still bought the PC version of Dragon Age Origins because it was clearly the best version and I had a system that could run it at max settings.  With the internet nowadays people know weeks and months ahead of time which versions of the games are better.  

And yes it's very common for people to prefer the exclusives from one hardware company over another but can you really claim that 1 console or the pc best games #1-5 is so good that even the 5th best game on that system is better than the competing systems #1 game?  Sure I can make a 360, PS3, Wii, PC etc top 5 lists all day but other systems are going to have at least one title that I prefer.  For example I am looking forward to the wrpg Fable 3 on the 360 and it would make a top 5 most wanted 360 list but I like the Witcher wrpg series on the pc better.  The fps killzone 3 would make a top 5 PS3 list but I like the Halo franchise better.  I am looking forward to GT 5 over any other racer at the moment etc. 

You get the picture no one system has the best fps, action adventure, racing, platformer, wrpg, jrpg etc.  

If you think so then you are hopelessly biased and probably should have just stuck to buying 1 system to start with. 

It's not about one console having the best 5 upcoming games, but the 5 games you are looking forward to most.  I have a Wii and a PS3 and I can honestly say that there are no Wii games that I am anticipating right now more than the 5 PS3 games I listed.  That can change as more information becomes available, but for right now the things I want from the system are too far away for me to really get excited about (only things that far away that I put on the list are games like Portal 2 and TLG which make up for being far away by being sequels/by the same team as two of my favorite games ever)

So yes there are situations where one can legitimately vote for games from a single system even if they own two or more.  It's all about what you're anticipating more and if your focus is on a single system most of your anticipation will go that way.

Just a few comments after looking at your list

1.  Infamous 2 (PS3)  So you think this is going to be a better action adventure title than The Legend of Zelda Skyword Sword ok but you will probably be in the minority

2.  Final Fantasy Versus 13 (PS3) You like jrpgs but instead of listing what many reviewers have already called a great rpg in Kingdom Hearts Birth By Sleep that comes out in September from the very same Nomura or a promising jrpg like the 3rd Birthday that will be out by the end of the year in Japan you choose to list FF vs 13 a jrpg that after 4 years still has no release date or real gameplay footage just to keep your list consistent with all PS3 titles.

3.  Little Big Planet 2 (PS3) Ok you like platformers but apparently just the ps3 kind since Kirby and Donkey Kong got no love from you.  But maybe you like the level creation aspects of the game so you get a pass on this one.

4.  Killzone 3 (PS3) ok you are part of the minority that thinks Killzone is better than Halo. 

5.  Gran Turismo 5 (PS3) the only choice that made the most sense to me on this list.

Every game you listed for the PS3 had an alternative that's just as good (excluding GT 5) that releases earlier.  Even if you prefer the PS3 titles over the other games I listed the fact that those good games from the same genre are releasing much sooner in many cases should mean that they would make it on to your list before those far out PS3 games.  And those are just a few examples I can think of many other titles that fit the genre of the PS3 titles you choose that are well respected and liked.

I'll admit that I can understand your list simply because you only have a PS3 and Wii according to your profile and some of the titles you picked like The Last Guardian are very unique. 

But when people own a PS3, PC, Wii, 360, PSP, and DS and all of their most wanted games are on 1 platform I think something is wrong.  So for example everyone knows the best place to play portal 2 is on the pc so if you owned a gaming pc and still put Portal 2 PS3 on your list I would seriously have to question what logical reason you would have for doing so. 

That can come down to something as simple as a preference of controller.  I play any game on the PS3 over the PC or Wii because that's the controller method I enjoy the most and am the most used to.

Well, you know, I own all three systems, but I'm only looking forward to 2 Wii games and they didn't make it into my Top 5, and I'm only looking forward to 1 X360 game, but it isn't in my Top 5 most anticipated games. I also anticipated some multiplatform games, but they are not among the Top5. It just has to do with personal taste in games IMO.

You know you can connect a ps3 controller to the pc right?  And then you would enjoy all of the advantages of the pc version in addition to having your favorite controller

Which is extra work for visual improvements I don't really care about much, plus having to set up my computer running through my TV so I can play on a nice big screen which means I can't surf the internet while I play.  Again, there are many reasons for not picking the platform that "everyone knows the best place to play portal 2 is on" beyond just being biased.