When someone says that a sales data for a game from VGChartz is likely undertracked or overtracked, how do this person know this?  Where is that person getting this information (objective data)?  Or not some anecdotal info from friends' friends that works as cashier at gamestop.....  Does this person sit in the CEO meeting room and know sales data for a certain game?

Sometimes it's (educated) guesses, sometimes we see the info from another company first, and sometimes we just like to say crap like that.

NPD, UK and EU chatz, BIU (Germany), PRs (with shipped data), Media Create/Famitsu (Japan), Financial numbers, etc.

There is a lot of information in the internet.

Some examples:

GT5 shipped 9,190k as of Sep 2012: http://www.polyphony.co.jp/english/list.html
VGC had it at 7,856k as of 29th Sep 2012: http://www.vgchartz.com/weekly/41182/Global/

1,334k on shelves for just a game is way to much... so undertracked... a old game is suposed to have ~500-700k units on shelves in this time of the year.

Either using data of software tracking firms like NPD, Gfk, Media Create etc. or by using official shipment numbers.

Good explano. You describe it really well (no im not being sarcastic).

Compare God of War 3 vgc vs God of War 3 official.

Don't take the numbers on this site seriously and you will be ok. We'll just call it magic for now and leave it at that.