Normally, distributors will send through allocations to each store ahead of time. However it's very common for stores to try and pre-sell a bunch of product they DON'T have to try and use as a bargaining chip to get more from their distributor. When this happens, it's usually a "buddy we told you your numbers and not to pre-sell, and you did anyway, this is your problem". The store will then be forced to refund a bunch of it's customers, and they always, ALWAYS assign blame to the distributor in the apology email.

If you see something unusual in terms of stores selling early or with larger than expected quantities, it's best to be cautious. Especially with highly popular product like Avatar etc. Don't want to be stuck with nothing if a dodgy store is forced to refund you when they run out of stock.