It is statistics for the 3 main regions including Japan. The vast majority of Japan users will use it in handheld mode.
Seems like a lot of the replies are saying they use it handheld but mainly inside their houses(toilet maybe lol?). This should be even easier for sony to accomplish as an optional accessory. I am pretty sure the vita could already stream your games from the PS4 via remote play. I think it just wasnt marketed that well plus it was expensive. They should make a cheaper version 99$ as an optional accesory to play your ps4 game in your toilet or wherever.