There was a statement about it somewhere, but you don't seriously believe it was designed with 4:3 as the default. The whole thing about the Xbox360 is 720p which is 16:9. Pretty much about every single game that's on the 360 is designed for 16:9. The same goes for the PS3 with 720p and 1080i/p. Both are 16:9.
Assuming that fov 80 was designed for 16:9 is wrong, since GoW is messed up on the X360 in exactly same way as Bioshock and UT3 Demo on PC. U can check out the epic gow forums, a guy made a comparison and on 4:3 there is more vertical info on X360.
Believeing isnt the right question here. Sure a guy would assume they program on a 720p machine, but most likely they still use a 22" CRT with 4:3 res. Thats in fact is the default FOV->ratio and not 16:9.
So u probably need to rethink your calc, since its just a calculated value.. Did u actualy test all those settings on a pc and compare the viewable angles? The guy who sad fov 90 felt better on a 16:9 is probably totally right. With your calc value it would be like 78 which is smaller than the default one, which would make no sense for me, since its zoomed in too much already.
Greetz