Do snow-dominated basins have lower or higher runoff ratios?

The recent Nature Climate Change paper by Berghuijs et al. analyzed the MOPEX catchments. Interestingly, they found that streamflow is greater for snowmelt-dominated catchments:

Fig. 1 from Berhuijs et al.

In their Fig 1b, they show that basins with snow:precipitation ratio of 0.6 vs. 0.2 would be expected to have about 10% more runoff. They don’t attempt to answer the question of why.