prastro Could it be a loading issue - at Dec close to 0, the scope body is more or less perpendicular to the RA axis, which closer to the pole, it is more parallel?
(1) When you did these measurements, what was the guide camera's angle? Is it possible to redo the measurements so that the declination axis is almost parallel to the camera's x- or y- axis?
All these measurements are measured by PHD2 using pixel coordinates, and they have to be rotated into equatorial coordinates by a matrix that is obtained during guide calibration. If the guide camera angle is rotated with respect to the declination axis, the off-diagonal terms of the rotation matrix are non-zero, and their accuracy can mask what we are seeing. For example, conflating a declination axis problem into an RA axis problem.
(2) For what its worth, there is one major difference between pier-west tracking and pier-east tracking. When you are tracking a target that rises in the east, the RA motor (and gears) is "resisting" (actually braking) the weight of the instrument on the way up to the Meridian. After the target has flipped to the west of the Meridian, the RA motor is now "pulling" with all its might to track the instrumentation towards the west. It is kind of counter-intuitive, but you can visualize it by loooking at the mount.
This happens to all mounts from Fraunhofer's original clockwork (why they are called "German" mounts) to the strain wave geared mounts today.
(3) You might also try to balance the instrumentation on the declination axis as well as you can. With my mount (RST-135), I place the OTA and its declination dovetail plate on a flat table, with a pencil underneath the plate as a fulcrum, to get a position where the OTA "balances" best, and mark that location. When I mount the OTA on the declination on the dovetail clamp, I would try to line that mark near the center of the clamp. I don't try to balance it perfectly, since a little off balance is actually a good thing to have -- but too much imbalance, and it might become a problem.
Chen