I reckon they're lowballing.
Each game they (and we) have expected to make a big splash hasn't (Mario 3D, Pikmin 3, Wii Fit, Wind Waker), so betting on Mario Kart 8 and Smash going nova is a little naïve.
If you break the forecast down (and this is speculation, I've not read their reports), I think they're probably saying 2 mil for Xmas, maybe .6 for Mario Kart and Smash and then 1 mil spread out over the year, which is an average of ~19k per week.
That is a very low estimate, if they had a forecast of say, 5 million (2.5 xmas, 1 MK/Smash, 1.5 rest - ~30k per week), which still looks fairly accurate, and missed it, then investors would be flipping out.
They've forecast low, so that they only need to adjust upwards.
I actually think it's a fairly honest forecast on Nintendo's part, probably knowing they can't keep BSing investors forever. They had to give an honest forecast this time around because Iwata has a shareholder's vote of confidence coming in June, he can't be seen to be lying to investors flat out.
They shipped 2.72 this past fiscal year, assumming Mario Kart 8 gives them a sizable bump of say 800k extra Wii Us sold and Smash U is about the equivalent of what Mario 3D World was last year ... that's probably a pretty fair assesment. Honestly they'll probably have to sweat to get to 3.6 mill.