There is a clear difference between subduction zones facing east (slope 27.1°) and west (slope 65.6°), on the average. The first reason to be examined is Earth’s rotation. Slab is subducted about 660 km, close to 1/10 of Earth’s radius, and therefore it loses rotational velocity on the way. As this happens very slowly, it might be overlooked, but this is actually what happens. All this mass loses considerable amount of kinetic energy to the environment during the process. This fact constantly alters the slab dip as it descends into the mantle. It occurs linearly, having rotational speed u=1 at the surface, and u=0.9 close to the depth of 660 km. The Earth’s radius is 6,370 km, and therefore we roughly say that when fully subducted it has lost 10% of original rotational velocity.
This is shown here with the drawings below. Two rather similar triangles appear, as it is supposed that the lithosphere plates subducted westwards and easwards are affected equally by the difference of rotational speed, only with opposite signs of plus and minus. The distance gap is shown with the short line of the triangle, connecting the red line of real flow, and the black lines of imaginary trend of no rotation. Those lines are found to be accurately in between 65.6 and 27.1, or about 46°.
The information about average dip of slab is from the article ´Polarized Plate Tectonics´ (2015), by Carlo Doglioni and Giuliano Panzax.