I just realized something, maybe Nintendo shifted their prediction to 17m because they believe that is a much easier goal to not only achieve but to also exceed (like say with sales of 18M) and has a more positive effect overall than forecasting 20m but being off by 2m.
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.