I think you might be asking the wrong person. I haven't been vociferous on this subject. I'm asking whether other people believe this to be true because their arguments seem to be leading this way.
But if that is where my argument was leading me, I would draw back and steer down a different road. I would assume my argument must be flawed somewhere.
Why? Because you only ever find or don't find what you test for. The results of the tests are shaped by the nature of the tests. The exams, the coursework, the verbal and non-verbal reasoning, whatever. And of course intellectual ability changes over a lifetime and so on and so forth. You know all this.
"Why can it not also bee possible that they are on average less intelligent?"
I suppose it can be. But it's not the obvious conclusion you jump too. If there were racial tests and one group came out worse, the initial and one hopes final assumption would be that they had had a poorer education, not that they were less intelligent.
I think your question is a bit redundant. It's only really relevant where no cause is ascribed to the underachievement of boys -- so it's up to you to answer it, because I have my reasons.