That really has nothing to do with the first three bosses who have already been killed.
I would imagine the data comes from the same place your achievements and statistics come from. Blizzard records your wipes on bosses ( which is why in statistics you can see what boss you've wiped the most on), it just doesn't show up in your statistics page.
Pretty coo, feature. I've noticed some weird discrepancies, but most were fixable by updating. People need to log off like they do for guild achievements.