The total memory available is the amount of memory left after the printer has set aside its required space to operate. The majority of this is the largest page buffer possible at the highest resolution, given the amount of memory installed in the printer.
For example, a LaserWriter Pro 630 with 8MB of RAM would allocate a page buffer for a 600dpi Legal-sized page (approximately 5MB). The LaserWriter Pro 810 with 8MB of memory would allocate a page buffer for an 800dpi Letter-sized page (approximately 6.8MB) since there isn't enough memory to create a page buffer for an 800dpi Legal-sized page (approximately 8.8MB).
In addition to the page buffer, the printer needs to allocate space for the PostScript interpreter itself, PostScript dictionaries, display list, font caching, and program memory.