That 40 Mhz crystal, then, may be driving your processor at a more sedate 20 Mhz. It's hard to build very high frequency crystals, so many of the speediest CPUs divide the input by 1. Some don't accept a crystal. Others multiply the input using a phase locked loop. This has two advantages.
Watch crystals are cheap and very small.
Even better, the 32.768 Khz input is ideal for tracking the time of day in the real time clock modules included on many of these parts. Count on continued confusion when comparing raw bus rates. Only the newest and fastest parts do anything useful in a single clock cycle. The 68332, for instance, needs 3 clocks to read anything, including an instruction, from memory. The 186 needs 4.