I'm pretty sure that the games with widescreen that do not fill up correctly, often (always?) have inaccurate aspect ratio and in fact are somewhat squized.
I belive this to be true with most Gamecube games (Burnout for example) but also Wii games such as Marvel ultimate alliance, Resident evil 4 and SSX Blur. Compared to the games played in 4:3 display with correct proportions on my 16:9 tv (pillarbox) the 16:9 mode in these games are significantly more narrow than in 4:3. Go on and check it out for your self.
In fact the picture is so much squished that it would not be enough to stretch only to fill up the screen, rahter it would be necessery to stretch it so much as to fill up the screen and normal overscan. Only then would the game have right proportions.
I surprised i have not seen anyone complaining about this anywhere. I am not the least bothered by that the games do not fill up correct. But i am furious about the wrong aspect ratio! Marvel ultimate alliance was outright unplayble - the charactes were so narrow and looked like a joke.
I think this is more than simply underscan, in that case we should have black bars on the bottom and top and that is only true for pal games (due to more lines in that standard - 576 instead of 480)
I cannot come up with any explaination WHY they make the games like this. I would very much appreciate if someone with some insight in making of games could explain it. I do understand why games are jaggie as hell, and why games sometimes use dithering or other substandard color tricks, or why there are a lot of tearing in the games. I also understand that it can be painful to make a 16:9 mode and why it is thus left out.
I do not however understand why they make these "15:9" games with squished aspect ratio. Because as we all know, some games works perfectly fine.