Here is a thread which you may find interesting on XP - which discusses voting a bit.

Also, you may want to go to some Saint's home node, go to the list of their nodes, and sort their nodes by reputation, high first, you may be surprised by some of the nodes that are among their top. Of course, it won't tell you what the reputations are, but you can get that their top ones are at least the ones that got the most votes.

Bottom line: nodes that contribute the most to PM's members get more votes. Nodes that contribute to PM, but only to a few members, may get fewer votes.