Are All 1964 Kennedy Halves 90% Silver?

yep

They are supposed to be. If you have one that really is 40% silver clad then be very very happy.

It just seems odd that that the mint would continue to make 90% silvers after the passing of the Coinage Act of 1965.

True, and I'm not complaining, 90 percent silver is wayyy better.

The official government line was that the silver and clad coins would co-circulate side by side for decades to come and there was no reason to hoard the silver coins. in order to maintain that fiction, and to encourage coin collectors not to hoard the coins because therewere too many of them to ever have a premium value, the 1964 date was frozen on all the silver coins and production was continued until there were enough clad coins produced so that if the silver coins were snatched up an hoarded once they stopped making them commerce could continue. If they had immediately stopped the silver coins when they started the clads the coin shortage already taking place would have become MUCH worse. As it was once they did finally stop silver coin production most of them were gone from circulation within a year.