The literature reports that permeability constants differ in experiments conducted with gases or liquids. This behavior, known as the Klinkenberg effect, has been explained based on the gas-slippage theory. We show here that inertial effects not considered by Darcy's law are the cause of variations in the gas permeability constant, k(gas) with the mean pressure, P-m. The dependence of k(gas) and P-m on fluid flow is fully explained by the proposed model, based on Forchheimer's equation. The use of the gas-slippage theory is unnecessary to explain variations in k(gas) obtained by Darcy's law.