Phase aberration in ultrasound images is caused by inaccurate information in the sound speed distribution in the medium and can result in image distortion, such as shape change and position shifting of the imaged objects. Various methods, including cross-correlation-based methods, have been applied to the distorted images to estimate phase aberration. In this paper, we first propose that the position shifting induced by the phase aberration causes the estimated phase aberration to be inaccurate. Then, we propose an equation relating the estimated phase aberration to the true one and the equations to predict position shifting. Finally, we present a forward model for estimating phase aberration. Considering phase aberration as a function of the array element position, the theory shows that both the constant term and linear term of the true phase aberration will be cancelled in the estimated phase aberration as they result solely in predictable position shifting. Field II simulations and data from tissue-mimicking phantom were used to validate the proposed theory. The theory was also applied to improve the estimation of the initial delay of ultrasound probes in both Field II simulations and experimental phantom study. Other potential applications of the proposed theory were also discussed.