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 the theory part of this paper, we first proposed that the position shifting induced by the phase aberration causes the phase aberration to be inaccurate. This paper proposes an equation to relate the estimated phase aberration to the true one as the basis of a forward model for estimating phase aberration. This paper then shows that the shift in focus point can be estimated from the true phase aberration. Lastly, this paper shows that the estimated phase aberration can be predicted or modelled from the true 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 mostly cancelled in the estimated phase aberration. However, a quadratic term and a cubic term in the estimated phase aberration will result from the true phase aberration's constant term and linear term, respectively. Field II simulations 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.