Me too. But different encoding software will also produce different results. The MP3 spec only tells what the format of the compressed audio file is and how it is decoded, but doesn't fully dictate how the encoder should be implemented. There are various patent issues around this and a number of algorithm variations that can be used. Usually most differences show up with small bit rates, and once you throw something like 320 kpbs at the problem, you'll be hard pressed to hear the differences between encoders (or the whole MP3 compression in general).
But after the encoding has been done, your MP3 file is what it is and there's only one correct way to decode it. Everything else is, as Puncher says, just some kind of effects that may improve on the perceived quality, but will add something that wasn't in the original music.