MP3 rips are not identical?

Just wondering - I use AudioGrabber with Lame encoder. Why would the files not be identical when I rip a track multiple times? It gets the digital data, encodes it using the same settings with the identical algorithm. The check-sums of the digital data are different. Could someone please explain why the files are not byte for byte identical? Is it because of read errors from the CD? DO all CDs normally have a LOT of read errors? Not a single track is the same! Thanks.