As a York/Luxaire dealer I can assure that York does indeed have 12 seer VS units.
With VS blowers there is a hum terminal, that you hook a humidistat to, when the humidity is higher then this set point the CFM is reduced to remove humidity.
When the humidity is lower, the VS goes to its higher cfm, to cool like most psc motors.
It also allows many people to set their stat higher since the himidity is lower.
Nothing wrong with 45% rh in summer.
Also on the milder temp days, the normal psc blower is moving the same cfm's as if it were 95 outside, so you don't remove as much humidity due to the short cooling cycle to maintain temp.
I take it you haven't installed many.