Jet rates in deep inelastic scattering: wherefrom comes the sensitivity to \alpha_s?

For theoretically consistent determination of ${\alpha }_s$ from jet rates in deep inelastic scattering the dependence on ${\alpha }_s$ of parton distribution functions is in principle as important as that of hard scattering cross-sections. For the kinematical region accessible at HERA we investigate in detail numerical importance of these two sources of the ${\alpha }_s$ dependence of jet rates.