The manufacturers’ recommendation might also have to do with tool life.
With a little simplification, the axial engagement ap for which exactly one edge is always engaged is, if I got my sketch right,
where D is tool diameter, n_f the number of flutes and \lambda_s the helix angle. This will “smooth out” the forces over one revolution. That gives:
2 flutes, 25° helix: ap/D = 3.4
2 flutes, 55° helix: ap/D = 1.1
3 flutes, 42° helix: ap/D = 1.2
That said, sometimes reality catches up with that theory when the pocket to cut ain’t that deep…