Part of the load on a tire is supported by the construction of the tire at the actual deflection of it.
So pressure on the ground is higher then the over-pressure inside the tire.
I estimate that part to be about between 5 and 15 % of the maximum load of a normal car tire , but can go as low as 1% of that for a Truck tire with high pressure .
Then if you would determine the surface on the ground and multiply it with the over-pressure the load on it calculated is lower then the real load on tire.
For truck tire difference is small ( probably that 1% construction load, as I babtised it), but for normal car tire difference will be about 5 to 15%.
Even the profile makes the pressure on ground higher , and if you would place the tire on a blunt nail bed the tire bends the same and by that the pressure on the nails will be much higher.
So if you would then determine the surface on ground by the number of nails depressed and compare that with normal underground, the calculated surface will be the same .
So its possible that the total load on a tire is only carried by the construction of the tire. then the real load on tire is about that 1 to 15% of the maximum load given on tire.
Then the tire has the same deflection as when maximum load and pressure needed for that on the tire .