These types of probabilities aren't absolute in the same way that flipping a coin has 50/50 odds.

Its more like a monty hall problem, each door either contains an asteroid collision, or proof that its not going to hit earth. But we dont have perfect information on the size, albedo, spin or orbit of the asteroid so most doors are closed. Each time we open a door and do not find the asteroid, the probability of finding proof of an earth collision AND a proof that its going to miss will go up.

Calculating a spacecraft/asteroid trajectory is already precise enough to land a mars rover in the correct target area. But we know the exact properties of the spacecraft so all the doors are open.