I think one of the issues recently is with the more restrictive cancellation policy (2 or 3 days prior), the oversell percentage stat does change a bit.
I have come across a number of properties at Hilton that wouldn't Diamond Force. I have called each time and the Diamond Desk confirmed the oversell rate was over the threshold to allow further bookings. Like Marriott, I can book Diamond availability on the website (and as indicated by others, Diamond reward bookings can also be done, but with the destruction of the reward chart, this is not so valuable).
Thankfully, I have not had to use Marriott Platinum Reservation much (as the rate is not great), but I would not be surprised if Marriott now has more defined parameters on how much oversell they allow as a recent enhancement on the website. As a Plat, I see quite a bit of sold out situations on the website which used to never be the case.
Rasheed