If they do...it must be one heck of a mathematical computation to figure out the probablity...then of course there's the differences of consequences of a direct hit, a negative rotation hit or a positive rotation hit...is it in the Northern Hemisphere during which season or the Southern?
I bought a Lotto ticket today...odds 41,600,000 to one...hundreds of people have "beat the odds."
There are two ways to figure a probability: Calculate it or estimate it. I think the "once every 100 million years" assessment falls into the latter category -- though we do have a much better idea what's out there nowadays than even 20 years ago and can do a better job calculating these probabilities. You're definitely correct, though, I was only referring to the "unconditional" probability of a 6+ mile bolide clobbering us.