Comment by Xirdus

Yes, but sometimes (quite often actually) you really don't care. The entire subfield of audiovisual compression is built around the principle that you don't care. The data is actively being lost and the image distorted, and you're fine with it. Wanting deterministic results out of MP3 is like wanting AC units to be loud. Sure, they are loud, just like MP3 is deterministic, but why exactly is being loud one of your requirements when choosing an AC unit? Isn't it enough to produce cool air, or in case of MP3 - sound good enough?