The system for XP gain and loss is documented in the link that appears underneath the Chatterbox (if you have it enabled) on every Perlmonks "page." => Voting/Experience System. Basic idea: your chance of gaining XP when you get a ++ on a node depends on how good the nodes have been lately.

Update I add further that, according to that table, it's harder to lose XP than it is to gain them. So you fell out on the bad end of Tyche's workings this once. It's not the end of the world anyhow =)

Trust us, it all works out. You can be the recipient of repeated downvotes on a number of nodes, and still show a net XP gain ...

Philosophy can be made out of anything. Or less -- Jerry A. Fodor