I suspect it has to do with two things.
First It's only a month. For comparison the Ps4 sold about 3 million, and the Xbox1 sold a little less. However this was during the holiday season, which translates to higher sales.
Second Create artificial demand. If I remember correctly, this happened with Amiibo to an extent, and with the Wii (however that wasn't artificial. Doing this encourages the consumer to be more excited for the product and willing to pay more. This means that the consumer that buys the Switch on launch, they consider it a premium product and will buy more additional stuff with it, such as more games/controllers. Also HYPE!
Third Late March release date. If its release on March 21, 2017 I doubt they'll sell 2 million in 24 hours.
Fourth Conservative expectations. It's easy to hope for 4 million launch sales, however Nintendo may be low balling it so that if sales are higher, its good news and investors are happy. If its lower, well that sucks.