There could be multiple explanations applicable here.
These are pre-orders; without knowing what the initial allocated total stock will be, we have no way of knowing what percentage of that is going to Amazon or even what number of units that translates to. SCE obviously can't take unlimited orders that have to be delivered on release day.
With a hot product, as was the case with the PS4, there will be a fair level of product flippers buying up supply, gambling on profiting with holiday secondary sales. If initial supply through 2016 is limited, this is a given.
So a fair amount of the initial buying is based purely upon speculation and VR hype. That's enough to drive demand to exceed Amazon's initial allocation.
But until SCE releases any sort of numbers, claiming that those initial allocations are small enough to belittle or conversely claim that PS VR will be the biggest hit of the holiday season for 2016 is way too premature.







