Fourth Clutch of Bluebird Eggs!

I was surprised to see these two eggs in the bluebird nest this late in the season. I have never had four sets of fledglings raised in a season...usually two and sometimes three.

Update: By September the eggs were still in the nest but no sign of any parents. There was no evidence of fowl play (pun intended) so I wonder what happened. The mother could have come to some misfortune outside of the vicinity of the nest but I am curious whether there could be some other explanation due to the lateness in the season. Four nestings are unheard of.