The forecasts aren't "what we think we can do", they are "what we think we *will* do". It seems that Nintendo decided to ship this many units, and so they did.
There shouldn't be any big differences between the last quarter's forecast and what happens during that quarter, since it would be quite bad if a company couldn't predict what will happen in the following three months. I'm guessing that that's why Sony took so long to reduce their unattainable forecast from 11m to 9.5m PS3s; they waited until the point when it would look too bad to miss the forecast.
Now you could ask, why did they decide to only ship this many units? The answer would probably be that they need to stockpile for various events: holidays (not just Christmas), new launches (notably China and South Korea), big upcoming games (especially Wii Fit which should tap a new demographic). Since they already had a stellar fiscal year, they didn't need to ship many hardware units in the last quarter, thus they can keep stock for when it matters more.