Do you mean pillerboxed rather than letterboxed? That may be where the confusion lies as the whole of TSFS will be letterboxed due to the aforementioned aspect ratio. Judging by the thumbnails of the screencaps over at TrekCore
there does seem to be some pillerboxing going on (harder to see when viewing each shot individually as all the excess black bars are cropped).
Comparing the DVD and Bluray screenshots however, it looks like possibly there was some horizontal stretching of the picture on the DVD?