Not 100% sure, but I believe the last time I did a cabin change (though was standard Y award to saver IN), rather then simply leave it at even (I believe the standard Y/Saver F rates - this was domestic - were equal) I had the full amount of the original award re-deposited and the full amount of the new award taken back out again. I'm pretty sure it happened simultaneously, if not fairly close to each other. So my guess is the same sort of thing happened here - bit you logged in during a moment where there was a lag between the full re-deposit and the debit of miles for the new award.