The physical origin of the recently reported wavelength scaling law of high harmonics generation (HHG) yield [Tate et al., Phys. Rev. Lett. 98, 013901 (2007); Shiner et al., Phys. Rev. Lett. 103, 073902 (2009)] is not fully understood. We re-investigate theoretically the wavelength (from 800 nm to 2000 nm) scaling of HHG yield in atoms with different soft-core Coulomb potentials. It is found that the power x of the wavelength scaling λ^x surprisingly oscillates in the region -3.63 ∼ -6.32. A similar oscillation is found in the wavelength scaling of ionization ratio upon the ionization potential, indicating that the wavelength scaling of HHG yield is closely relevant to the ionization. The origin of this oscillation in ionization is also identified.