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.
That's a strange way of looking at it. It's like you think investors have Alzheimer's or are Trump stans.
In Jan-Mar Quarter of 2018, Nintendo shipped 2.93 million and so far this financial year their non-holiday quarters have been less than stellar.
So adding 2.93 to 14.49 million would have given them 17.4 million. and expecting them to hit 18 million meant that they have to beat last year results by 20%, which is not an easy thing to do especially if there is no major software release on the horizon.
So i think they have just been cautious which is a good thing at this point.