I've seen this before a couple of times even for mainline. Whether could certainly be a factor in why they haven't scheduled a particular aircraft yet given there will probably be planes/crew out of position. Though I've seen this too on days with little irrops to speak of. So nothing odd.

Just so you know, even when there is a specific plane listed, within 1-2 days before flight, it often changes anyway. In fact, I'd say more often than not.