Since AMD only started selling their own(Well, Sapphires) cards again recently I'd assume the gap they usually have is because they want to get the cards to market quicker than would be possible if they waited for OEM partners to manufacture and ship out enough custom stock for launch. A 1 month delay means the custom cards must have been completely finalised for launch day, maybe Sapphire just get the bulk of the early chips until closer to launch day so AMD knows they can meet OEM PC manufacturers demands quickly. Of course NVidia has had no reason to rush any of their recent launches really, though with the RTX2080/2070 they were essentially paper launches with barely any stock at all for weeks if I remember correctly.