I think you suspected right in the first instance. The "block" share was found on slush ..... but it was stale, i.e submitted after the work was timed-out (or after someone else had already submitted the same share)!

On the stale share being logged as a found block, I believe that is a bug in cgminer (or it may indeed be by design!), but the best place to ask that question would be in the cgminer thread. I say it may be a bug because despite clearly not being logged as your best share (and is definitely higher in size than your best share), it has been marked (and logged) as a found block, despite it being stale. So yeah, head over to ckolivas' thread and throw that his way ....

edit: Very strange as you seem to have good connectivity to the pool, aka only 1 stale share in 2 days + ...... very strange indeed that it had to be the found block that turned out to be timed-out!