That makes a lot of sense. A lot of aviation power systems run with 400 Hz AC current (the higher frequency lets them use smaller transformers). They could be dividing down the power signal to 100 Hz, and using that to increment a counter.
The other option is that many operating systems use 10 ms = 100 Hz for their internal interrupt timers. So it could just be a counter that is being incremented every interrupt cycle, and doesn't care what frequency of electricity is being used.
(cf. the jiffy http://en.wikipedia.org/wiki/Jiffy_(time) )