In short: public health isn't about you, but about protecting everyone else from you.

As the probability of the path of the asteroid gets refined with time, the uncertainty funnel gets narrower. That results, mathematically, in a smaller funnel. Naturally the Earth becoming a larger and larger portion of the probable path. The metric makes no difference in the actual probability.

This is the org that:
* "forgot" to tell anyone there wasn't fluoride in the water for more than a year
* claimed that releasing the location of fire hydrants would facilitate terrorism
* sent me vague letters about a meter replacement with a logo so poorly reproduced I was sure it was a scam.