That still doesn't mean you can just add the percentages. You cannot determine the percent of boys+girls soccer head injuries without knowing the sample sizes.

Or, if you can figure out the relative difference between the number of boys and girls who play soccer, you can do a weighted average of the percentages.

Either way, adding the percentages is completely incorrect.

EDIT: http://www.usyouthsoccer.org/media_kit/keystatistics/ suggests that the split is roughly equal -- in 2008 it was 52% boys and 48% girls.

With that said, the correct weighted value is more like 21.5x.48 + 15.4x.52 = 18.3%, which is significantly less than the 40.5% you quoted for football.