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!
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Hint; Do a search and find our what is the only difference between an F250 and an F250 HD
mike hunt

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Mike Hunter wrote:

Or an F-150 & an F-150 "7700"... or a 1997 F-150 & a 1997 F-250...
Rob

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Psssst... count the wheel studs on a "7700".... This is the old F250LD...
Confusing, yes....

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