An important starting point is, did your surgeon measure in detail all of the natural asymmetries and differences that exist in all women's breasts? Perfect symmetry does not exist on anybody, and natural differences between the left and right sides are perfectly natural on the breasts, and also on the face. That being said, it is normal for breast implants to settle at different rates between the left and right sides during the early post-operative period. If a patient develops Grade 3-4 capsular contracture on one side, then this will definitely distort the breast and cause asymmetry. Finally, if there were significant differences between the left and right sides noted pre-operatively, then different strategies such as different sized implants or more complex augmentation mastopexy with breast reduction and implants should have been discussed.
I hope this helps.