Why does Quicktime clip tails of Mp3 files?

I work in audio and am often frustrated by the fact that Quicktime always clips the tails (cuts the ends short) of Mp3 files. I go to play a 60 second Mp3 file, or a 120 second file as created on Audition, and Quicktime cuts it to :57, or :115, or thereabouts... Every time! I have found no answers to this problem anywhere and would love some help or advice! What concerns me is that the files I am sending are clipped by some other users, too, including the folks who review my audio. Other players do not seem to have this problem. Thanks!