The 100 MHz is a marketing thing. When Apple discovered that th ecompetition talked about the coreclock to make its machines sound faster they eventually jumped on in (and so did 3rd party producers).
The 68040 doubles the clock internally. The core of a Blizzard 1240 40 MHz really runns at 80 MHz. So the 100 MHz thing is just an other name for an 68040 clocked at 50 MHz, and that is about how fast an 68040 could go without nytrogenecooling.
