The total memory value in "About this computer" (7.6 and later) and "About this Macintosh"(7.1.2 - 7.5.x) is obtained from the "lram" (logical RAM) Gestalt selector. This number is calculated slightly different on the Power Macintosh due to memory used by the NanoKernel (68K emulator). An adjustment to the number is performed to yield the best possible estimate of the remaining RAM as shown in the value for the largest unused block.
The discrepancy in values returned from different machines are usually caused by various color depth settings for on-board video. The expected discrepancy for the NanoKernel should be around 870K.