When you change the Stop location, the Exit Pupil location (and size) also changes.  If aberration is present, then a little thought will reveal the fact that, as rays are traced from the image plane to the exit pupil reference sphere, the OPD in the exit pupil will also change.  Hence the Zernike fit to the OPD wavefront error will be different (the amount of variation depends on how many terms are used for the fit).  The greater the underlying lens aberration, the more the wavefront error will vary with stop location.  To monitor spherical aberration, try instead using the SPHA operand (third-order Seidel spherical aberration).

Here is a simple example using a biconvex lens.

Let’s set up two configurations, one having the stop 1 mm in front of the lens and the second with the stop placed 100 mm in front.  We see that both the OPD and the corresponding Zernike fit depend on stop location, while the value of the SPHA operand remains constant.