This is due to a difference in the way data is stored on Audio vs. most other types of cd-roms.
In particular, a data cd has a some data integrity features that are lacking on an Audio ceedee (leaving a little less room for actual data).
Do not ask me, however, as to the nature of the beast. It might have to do something with redundant tracks (physically), or more spacing between tracks (meaning, again, physical, the nm between two adjoining tracks -if one can speak of those, since a cd is written as one spiral, or maybe that's just an audio cd, and a data cd is written as a hard-drive in concentric circles, divided in segments).
Anyway, as you can see: I don't know the low-level materialization of this.
However, I thought all cd's said: 700 MB (80 min.) and/or 650 MB (74 min.)?
It is really only time that counts with an audio cd. I bet you won't fit 82 minutes on a 700 MB cd, but you will fit 79 without a problem (totalling, oddly enough, a near 800 MB).
Maybe someone else can explain what happens on a lower level, for it is not a unintriguing question.