2005 F150 Payload Capacity Question

So the simple way to calculate payload capacity is GVWR - Curb Weight. Going by the specs on my 2005 F150 Lariat 4x4, this means I should have a theoretical payload capacity of 7200lbs - 5577lbs. = 1623lbs. However, there's a "loading" sticker which specifies a maximum weight of passengers + cargo at 1288lbs. Is this a physical limitation of the axle? tires? Why the discrepency?

Thanks!