[gravitronic]

From what I understand--

The "lossy" part of MP3 is that the encoder decides to throw away frequency bands to reduce the information size.

This is more than just choosing a bitrate and being done with it - different encoders treat different bands as important or unimportant, attempting to make a smaller sized file still sound better.

[weff]

Indeed but really, how many encoders does one use? That is, it's not like there are dozens of them that are widely used. If there are only 2 or 3 (say, LAME, some Apple one and some Microsoft one) and people usually use 2 or 3 bitrates... well that leaves quite the space for overlapping.

[pyre]

Right, but the music pirates don't write their own encoders. They are likely using software like LAME, EAC, and FLAC when ripping. Therefore, as long as someone uses the same settings, they could end up with an exact duplicate rip.