This paper presents a theoretical study of the loss difference between the left and right elliptically polarized modes in differential laser gyro (DILAG), which is caused by the S-P phase and Q anisotropy of the cavity mirrors and the off-axial propagation of the laser in the quartz crystal. This loss difference could cause unexpected null-shifts of the DILAG and the study about it would be an important reference in improving the DILAG operation.