I was quite alarmed at first but after poking around a bit it appears that their web programmers made a simple error -- interpreting "since last payment" as "since the date on which the last payment was made" rather than "since the time period for which the last payment was made". That seems to be confirmed here: http://hubpages.com/forum/topic/16072#post220410 Just a temporary glitch as far as I'm concerned until I'm shown otherwise.
I think for most of the adsense users (including me!), the missing dollars were added back into the account. It is happy to note that the glitch is fixed at last