Yup, I think it's at least to some extent a statistical thing. Girls that have the opportunity to get into riding at an early age are a very few & far-between occurrence, and simply limits the pool of talent greatly. But at the extreme of the envelope, there is a physical aspect to it which perhaps can be a limiting factor - at top-level race speeds, wrestling a bike around can require a lot of strength which may generally give men an edge. Then again, especially at that level, I think the extreme degree of competitiveness along with maybe a higher willingness to risk life & limb is more a masculine trait & helps explain male dominance of motorsports in general.
But back to apples, talking mere mortal street riders - I still think women have much more of a tendency to approach riding & pushing themselves with much more common sense & caution. IMO, while that probably helps many survive, it often limits the progression of their skills somewhat since they're less likely to push their envelopes & put themselves into 'do or die' situations than we testosterone-addled boys. Generalized & rambling, of course...