This is because the second Warriors Orochi game, which is titled Musou Orochi: Rebrith of the Demon Lord in Japan, was released as Warriors Orochi 2 in the west.
Koei Tecmo and Omega Force have announced Warriors Orochi 4, the first brand new numbered entry in the series since Warriors Orochi 3 in 2011.
Like the Dynasty Warriors series, the numbering of the Warriors Orochi series is one digit higher in the west than it is in Japan.
The actual Musou Orochi 2 was released as Warriors Orochi 3 in the west.