First, credit for this question goes to Sir William Arbuthnot, who posted it as a comment on my profile. I'm not sure why he picked me to ask, since I don't know a whole lot, but it seems like a very good question, so I'm putting it here.
I have observed that when first and middle name are both in the first name field, it fouls up searching. If someone's first name is "Mary Anne", when you put either Mary or Anne in first name, the hit list will not include Mary Anne.
If, however, Mary is in first name field and Anne is in middle name field, then the person will be on the hit list for a search of either Mary or Anne.