The reason is whenever the energy input of the cylinder is not sufficient to gas the fuel you have to put more fuel into the cylinder so that the light fraction of fuel that gasses easier, makes up a combustible mixture. Its the same thing as when your engine is cold
This means the 'AFR' with best power curve using the cast iron manifolds may now be different using the headers. One more reason that shooting for a WBO2 reading is a bad idea.
Another thing is the drag strip run was made with the engine cold. This is pretty common with dragstrip runs. They never get heat soaked, and in this case it was dead cold.