This is perfectly normal for many DVDs...
Here's the deal:
Your television is 16:9 - it is not widescreen, it is simply 16:9 or 1.85:1
Movies, quite often, are a much wider format than 1.85:1 - They are often 2.35:1 which is considerably wider than 1.85:1.
So, you end up with a letterboxing effect when you view those DVDs on your 1.85:1 television. This is not always the case as some movies are put on DVD in a 1.85:1 ratio, but most preserve the original wider format that the original producers intended and you get 100% of the image the filmakers intended you to see.
HDTV is almost 1.85:1 (actually it is 1.78:1) so your 16:9 native television does a little processing and presents HDTV material perfectly on your set. This is really what your television was designed for. It was designed for HDTV which provides the best possible image you can enjoy, not DVDs which are substantially lower in quality than HDTV.
Letterboxing is minimilized by your widescreen setup though, so it is much better to watch them on your 16:9 display than it would be on a 4:3 display (1.33:1).
The BACK of DVDs will often say what the actual aspect ratio of the film is that is encoded on it - let me grab one for example: CASTAWAY is listed as anamorphic widescreen 2.35:1 which means, as said above, it is wider than my 16:9 television but shows everything that I would have seen in the theater. Now: Cat In The Hat is listed as 1.85:1 on the back... this means it will fit my TV (or yours) perfectly!
It is DVD dependant and has nothing to do with your display.