You do need a specific type of syringe in order to extract the 6th dose correctly and it was not the case that 100% of the distribution was set up with that particular syringe in mind.
Initially some doses were wasted because it was unclear whether that was a legitimate thing to do.
Then, after explicit guidance, they said "OK, these count as 6 doses, then, for the purpose of our Federal contract!"
Then, Feds scrambled to ship supplies to ensure low-dead-space syringes were being used everywhere.
That feels like one of the points where bureaucracy is stupidly killing people.
Having bad outcomes from pooling and not knowing whether it came from the combining practice, a certain vial, an entire lot, or even worse-- the fundamental vaccine-- could take the entire vaccination program off the rails. That's not worth it to stretch supplies a few percent.