Yeah there might be a difference, I have no clue. My take was large based on my own experience with having the occasional girl in my team as a kid, and up until about 12 they could keep up. The girls that played in coed teams were usually also much better players than the girls that just played in girls teams.
A lack of girls-only teams was definitely the core of it. I played with girls too but when more and more girls started to play football every club moved to girls-only teams. When I first started playing football my club only had 2 or 3 girls in every year group, when my younger sister started playing football they maybe had about 10-15.