Isn't this the same thing?
Best available, means best available at the time you check in (or during your stay, they do sometimes offer you an upgrade for some portion of your stay). Do you expect they'll bump some other punter out of his deluxe room because a Diamond has rocked up at reception demanding his inalienable rights to an upgrade? I wish that were the case

By the way, on the DD, I had a problem last year (of my own making actually) and called the DD. They weren't particularly helpful so I pushed it a little bit and was put through to some in customer care at 'corporate'. The service was outstanding. Not sure how or why that happened or if it happens everytime you express dissatisfaction with the DD.