Flight times and reward seats

I'm looking at reward flights for East and West Coast this year.
For the destinations with more than one flight SFO, NY it seems some flights have more reward availability than others.

SFO for example the later flight seems to have less availability. Is there some reason for this or is it just chance?

If the later flight time is more popular for cash bookings then it would make sense for VS to have fewer reward seats on a flight like that and more on the less popular one.

Same with NYC it appears there that some flights have better reward availability that others.