The key difference that people ignore is the installed userbase. Let me explain -
Lets say we have 2 systems, system A has 20 million units WW sold, and system B has 10 million WW.
Lets say a game comes out for both systems. In order to match sales, system B's version has to sell to 100% of the userbase to be even with system A selling the game to 50% of the userbase.
Now let's factor in development costs. System A costs 5 million dollars to make a game, System B costs 10 million dollars to make a game. For argument's sake, lets say that both systems sell their game for 10 dollars each.
System A sells to 50% of its userbase, making a gross income of 100 million dollars, minus 5 million for development, making their profit be 95 million dollars.
System B sells to 100% of its userbase, making a gross income of 100 million dollars, minus 10 million for development, making their profit be 90 million dollars.
At the end of the day, the game only has to reach about 48% of its userbase to make the same money as it would if it sold to 100% of the userbase of system B.
To simplify, a game could be a "flop" on system A and still carry a bigger profit than a "hit" game on system B. At the end of the day, it's only partially a popularity contest for a game, but moreso a means of making money. Developers don't go out and build games for the platforms a bunch of gamer geeks think is "cool". They go where the userbase is, which is why 3rd party developers are actually doing quite well on the Wii, even if their games aren't in the top 10.
We often rely on the top 10 lists, but we really should be looking at the actual money made on sales to see how well a game is doing.