Based on complaints I've heard (mostly from Marco Arment, on podcasts he guests on), the ad-injection platforms are often responsible for horribly mangling the mp3 format. They tend to not update the mp3 file's stated length, resulting in time-remaining being off if you trust it.

Plus, of course, dynamic ad lengths completely break the ability to share timestamps for podcasts -- no "you should really listen to the bit starting at 17:52"...