Although CDs are digital there may still be some loss involved in a burn/rip cycle. This is because audio CDs are less fussy about data correction than data CDs.

Even a single bad bit can be enough to ruin a data CD, so there is a lot of redundant information encoded on the disc for data CDs.

Audio CDs, on the other hand, are not so sensitive to data errors (think about how badly a disc can be scratched before it becomes unplayable) and do not dedicate as much disc space to redundancy and data correction.

To see this in action for yourself do the following: start with an AIFF file and burn it once to a data CD and once to an audio CD; then copy the AIFF file back from the data CD and rip it from the audio CD. If you perform checksums (for example, with md5) on the data CD and on the AIFF file that you copied from it you'll find that the checksums are identical. If you do the same on the audio CD and the rip from the audio CD you'll most likely see a discrepancy.

Actually, Audio CDs have a ton of error correction on them too. The difference between Audio CDs and Data CDs is that if you are playing an Audio CD, the CD player can't go back and re-read the data if it made a mistake the first time, because there'd be a break in the audio. Unless, of course, you use a fast drive and some buffering, like in high-end Meridian CD players.

Reading from Data CDs, on the other hand, doesn't usually have any real-time constraints, so the drive can go back and re-read data that it got wrong. Computer CD drives usually read Audio CDs in the real-time constrained mode (don't go back and re-read), unless you use a special program, or tick the "use error correction" tick box in iTunes. This usually means that it'll take longer for the CD to be ripped.