"The decay widths have higher-order perturbative QCD corrections, and these are particularly important for the decay which dominates over a wide range of (light) Higgs masses. The main effect is to cause the quark mass to run from its "constituent quark" value at to a lower value at ."
R. Ellis, W. Stirling, and B. Webber, "QCD and Collider Physics", Cambridge UP 1996.
Compare to the graph above, which shows the branching fraction of the Higgs boson to several final states, including charm, bottom, and tau pairs. Why is the tau-pair final state favoured with respect to the charm-pair one, despite the fact that the colour factor makes the charm pair decay three times as frequent, other things being equal ?