Think of how many we had in the last thirty years - Northridge, Landers, Loma Prieta, Hector Mines, maybe Cape Mendocino (I'm not sure if that counts). Imperial Valley and Superstition Hills were shortly before the window.

The odds do seem high, however. I suspect they are assuming all the faults' activity is uncorrelated, when we strongly suspect entire regions can be more or less active en masse.

I haven't studied the report, perhaps they are considering the northern and southern San Andreas faults somewhat "overdue".