But the original question is still unanswered.

On thinking about it, I'm guessing the airline's simply make less award inventory available coming the other way. I don't know why, unless they figure they can suck people in going over, and then squeeze more miles out of them to get back home. Or something like that.

In theory, it should be pretty much a closed system, with just as many going over as coming back (+/- 1% or so), so what would be the rationale for restricting availability one way?

'Tis a puzzlement.