High management of the biggest chains could be making unpredictable shipments to prevent or reduce hoarding. While hoarding is HW sales anyway, for stores it isn't as good as selling the units directly to each real end-user (*), because until the hoarder manages to resell them, they won't produce SW sales, that very often give them higher profit margins.
(*) in a slower period, hoarding would be better than having the units sitting on the shelves, but obviously hoarding only happens when demand is higher than offer, so those units wouldn't sit on the shelves at all.