Only a guess, but perhaps some are in mp3pro format which also uses the same file extension of .mp3
If they are less than 200 kbps in bitrate then a reasonable chance the person used mp3pro format to encode them. mp3pro format is good for encoding files at very low bitrates below 160 kbps (eg: 64-128), but that's all. Some players are not licensed to play mp3pro format which 'is' a patent format.