They never put all of the premiums up for sale, only a trickle at a time, so those could be held for future premium seating. And I've seen many shows not release them as quickly as the others sell out, even though one would imagine that could just be an automated system.
For your dates, they could also be holding a lot of those for reviewers, since they come a few days before opening.
Yea, often they hold the best seats for on sale closer to the date. They usually give pre-sale and early sales the worst seats. Which is weird, because the people most wanting to see the show get the worst seats. I've stopped buying ahead, with some exceptions and usually hit box offices the week I'm visiting. I get the best seats and pay no fees.
"The sexual energy between the mother and son really concerns me!"-random woman behind me at Next to Normal
"I want to meet him after and bang him!"-random woman who exposed her breasts at Rock of Ages, referring to James Carpinello