Here's my theory: the impressionist, though highly skilled, is relying on your existing knowledge of the characters to fill out his impression and make it seem more realistic. When you don't know who you're supposed to be thinking of, the impression is not as strong and it is harder to know who he is impersonating. This is not to detract from his incredible performance, just wondering how much of the impression we as audience members are providing ourselves.
Anyway, take the test and let me know which ones you got before looking at the answer key. My wife got three on her own.