The adjustable magnification object wavefront reconstruction algorithm effectively uses the space bandwidth product of digital holograms, but also introduces phase distortion. Mathematical expressions of the distortion phase correspond to each adjustable magnification algorithm are derived from present studies. The results indicate that a quadratic phase distortion superimpose on the magnified object wavefront, no matter the wavefront is reconstructed from Fresnel or lensless Fourier transform digital holograms. The phase distortions of + 1 and -1 orders constructed wavefronts are conjugate complexes to each other. The phase distortions is generated by a spherical wave, and the radius of the wave is irrelevant to algorithm selection, but mainly determined by magnification and the object distance.