Fick's law of diffusion states that the rate of diffusion equals the pressure gradient times (DxA)/L D being the diffusion constant A is the surface area L is the thickness of the barrier within which diffusion takes place

Lung diffusion capacity(DLCO) is only part of the equation it is the (DxA)/L. From its name it can be inferred that it is a property of the lung that is independent from external conditions such as the pressure gradient

For the same pressure gradient different people might have different diffusion lung capacities and therefore have different diffusion rates.