Back splashes can be warped and walls can be bowed either way. The proper way to handle these deviations is to use a compas and use the needle end as your deviation guide then the pencil will mark the deviations on the back splash. Then you cut outt his area and you will have a congruent match.
If the warp is too severe, then you need a new counter top with back splash. Hard to access being sight unseen project.