Danny:

It's not unusual but usually it takes pruning to force those later fruits. Usually the late fruits are smaller berries, smaller clusters, tighter clusters, and they don't get as sweet if ripening is delayed until cool weather.

Mid season pruning forces some of the buds that would have provided next years crop. So those late clusters are really next years crop come early. But it shouldn't reduce next years crop. A grape vine has plenty of fruit buds.

Thanks for the response. This is very helpful to know. It's odd, because I didn't do any mid-season pruning. I'm just letting it grow (which it is, with a vengeance).

I've got a vine that does this too. I was propagated by burial-snip from the main plant more than 2 years ago. For whatever reason the fruit on it matures about 2 weeks behind the main vines on the original plant. It gives me a little break with making jellies and wine as I don't need to pick everything at once before the wasps start poking holes in the fruit to drink the sweet juices.