@ expertno.1: There is also ABR! (Average Bitrate).
And im pretty damn sure that the discrepancy in the playing time is due to VBR.
Ok... lemme explain.
say u have an MP3 encoded @ 128 kbps. Then, for every second of audio in the track, the file will take 128 kilo bits. Thus you can calculate the playing time by knowing the file size. For example, if the file size is 1280 kilo bits, then the playing time is 10 seconds. This is the case of CBR (Constant Bitrate).
In case of VBR, the bitrate keeps on changing, so you cannot be very sure of the playing time, by looking at the file size.