My wife and I flew AC for the first time in quite a few years 2 weeks ago and we both immediately noticed this. It is indeed the reverse of what most U.S. carriers state.

I think the idea is this: they want you to at least try to put your larger item underneath, smaller one up top, thus increasing the chance that everybody gets to stow their bags and nobody has to gate-check. Reality is that for a lot of people it simply won't work - my rollaboard isn't going under the seat no matter what they say. But maybe for a few it does, and that frees up a little room for others and less gate-checking has to be done.

From a safety perspective, I didn't notice any difference. FA's are still going to make sure the bag actually does fit under your seat, and they're still going to make sure all bins are fully closed.