Not sure, but I've noticed the same thing here. It might be something to do with how much water the tree is getting, that it grows and blooms differently. If a tree is in an area that gets some irrigation it would probably bloom differently than one that doesn't get any water except the rain.
Here the Jacarandas bloom usually in April and the bloom is much longer and more luxuriant if there's been a fair amount of rain. This year most of the trees bloomed sparsely for a pretty short time. We had 8 months of drought and it finally just started to rain this week.
