That's a very good point. This seems to be a major point against an explanation based solely on encouragement/bias.

In a solely encouragement/discouragement based theory, it might be that for some reason, males are encouraged as well as discouraged more strongly than females. I don't know of any (even anecdotal) evidence of this happening, so I wouldn't bet on it.

In any case, the underlying origins of these statistics are no doubt complex, messy and controversial.