This article on asteroids hitting Earth contained this interesting passage:

An asteroid, or several asteroids, are believed to have kicked up so much dust and set off so much volcanic activity 65 million years ago that the resulting climate change wiped out the dinosaurs.

But scientists believe an event of that size would only occur about every 700,000 years on average.

Every 700,000 years? Now, I'm not the best mathematician, but doesn't that mean we should have had approximately 92 such catesrophic events in the past 65 million years?