When we are choosing which action to take, one of the most basic calculations which guide us is, "How likely is it to lead to one option or another." We need to think of all possible outcomes, and the rough probability of each one occurring. There is a problem with this. We are not great at assessing probability. But there are certain ways to make us think that one outcome is more probable than another.
Add a little description, especially when that description fits in with predetermined pictures, and people will chuck probability out the window. For example, give most people a choice between "Bob owns three guns" and "Bob owns three guns and hunts deer," and they will likely choose the latter as the more likely option. This can't possibly be true. Both Bobs own three guns, and imposing more conditions on one of them only makes their situation less probable. But it fits in with a stereotype of gun owners, and so people believe it "more."