I don't think this is too difficult to understand. Both in both situations, the deciders don't want to be think of themselves as possibly responsible for avoidable death. In the first scenario, you don't want to be the guy who made a gamble and everyone dies. In the second, you don't want to choose for 100 people to die. People make different choices in the two situations because they want to minimize moral culpability.
Is that rational? Strictly speaking, mabye not. Is it human? Absolutely!