This could be partially due to the idea of safety margins. If I meet something that I know very little about, I give it a wide safety margin, and get nervous if I am closer to it than my safety margin allows; once I know more about it, I can narrow down my safety margins considerably.

For example, if I know very little about snakes except that some are poisonous, and I happen to find a house snake in my house, my reaction would be one of caution (or, if I come across it very suddenly, even of fear) - with the aim of extricating myself from the situation unbitten. However, if I know enough about snakes to identify the snake as a member of a nonvenomous species, which apparently make reasonable pets, and that their first reaction to danger is to flee, then I would be a lot less afraid of the snake in question (though a cobra would still trigger a panic-run-away response).