Most TVs overscan the image. The main purpose of this is to eleviate the wide open standards of the broadcasting companies. Here's a WIKI post on overscaning worth a read:
http://en.wikipedia.org/wiki/Overscan
The other thing are that manufactures are very devious in their specifications. One will say that their set is HDTV ready. You would think that this means that the set is capable of displaying 1080i or 720p. Well it may accept that resolution, only to downconvert the resolution to what the TV can actually display (Samsung's are notorious for this). Here's a
.pdf specification of one of their TV's. Though it can accept 1080i, if you look at the first line
IN BOLD it indicates its max resolution is 1366x768...which doesn't quite work out to 16x9 (close by rounding numbers).
It not unusual to have to tweak your video card settings to display images properly. nVidia has this built in, and I think ATI Catalyst drivers have multi-resolutions and tweaks built in as well. You can also use Powerstrip, which works rather nicely to further tweak the image.