I believe this has to do with the choice of "Write Disc" or "Write Session". I'm still kinda new to this burning thing, and have only re-wrote to cd-rw's.
If you choose "Write Disc" in Toast, thats it for that disc. "Write Session" allows you to continue burn until you disc is full.
That's always the way I've understood it, I'm sure some one more knowlegable will chime in if I screwed up.
Justin