Forecasts have much more to do with production imo, as well as their book keeping. They only plan to produce and ship 2.5m more for the rest of their FY. They won't manufacture significantly more or less than 17m. That means if sales all of a sudden skyrocketed to the point that they would need to produce 19-21m, we would see major stock problems.
But aren't forecasts based on how much of your product retailers are willing to carry? So I would assume it's about production and orders equally. But you're probably right, lol. I just find it odd that the Switch forecasts have changed so much, but maybe that's normal for consoles. In 2017 I think it was originally around 14M and it changed to 16.5M, I believe because demand was beyond what was expected.