Oricon is a national reporting service, similar to soundscan in the States and Canada. Tohan is a single wholesaler. Consider Oricon more accurate. Please, post a link to where you saw higher numbers from Tohan. I am interested. Preferably in this
thread. Keep in mind this is not the first press, this is first week sales. By the time volume 48 comes out volume 47 will have sold 1.2 million copies or more. It's just they don't all ship in the first week. 750,000 is about Naruto's average for first week sales.
Anime News Network uses Oricons sales figures.