I've noticed around here that assuming they were the same species, the ones in more shade will bloom later (and the blooms will last a little longer) than the ones in more sun. Possibly related to the temperature in those spots too, as shrub flowering tends to last longer in cooler, less sun/overcast weather.

there are early, mid and late season azaleas. most of them are long gone here but i have some just coming into full flower now ('gumpo', 'wakaebisu' and 'macrantha') that are good for extending the season. the ones you are seeing might be those that typically flower later.