I have two elementary-aged children, a boy and a girl. They fit the pattern you describe to a T, and I think most teachers' experience would agree.

So anecdotally, the theory that (in general) boys need more nudging than girls from someone in their life in order to be successful rings true. But what would account for a modern change? Are there fewer people pushing, now? Are boys more resistant to it now, or maybe more distracted? Are expectations of boys different now than they were in the past?