I agree with your comments on the apparent lack of rhyme or reason concerning how Ancestry's search engine determines degree of relevance. It's frustrating to have to wade through 20 pages of results, rather than giving up after 150 items or so, simply because you know there's a good chance that the item you would consider most relevant is thought by Ancestry to be less significant and, thus, is way down the list. I often use my brower's "Find on this Page" feature against the results, but Ancestry's limit of 50 items at a time slows even that down.
It would be great if they could show us their engine's decision chart. It's a case of the old "I can play the game if I know the rules" thing. Even better would be the capability for the user to then rearrange the rules in order to produce results that make more sense to him or her.