The GPU core count above is a bit of "Apple's to Oranges". The baseline of the A10 era SoCs still had a design mainly based on Imagination Technology GPU ( PowerVR) . Apple had replaced bits of the overall implementation but it was still in the phase where was trying to appear as a ImagTech GPU. At A11 Apple shifted over to doing their own more explicit implementation. When they did they changed the appoarch of how they count 'cores'. So in realm of comparing AMD 'CU's to Nvidia ' CUDA cores'.
The A10 had 6 'cores' and A10X has 12 'cores'
The A12 has 4 GPU 'cores' and the A12Z has 8 GPU 'cores'.
Apple is doing substantive changes on each GPU iteration. The "cheaper to make" is easier to tag in a common generation. The A12 is smaller than the A12X so is less expensive to produce . ( even before take into account that the A12 has other volume Apple products to share production costs with and A12Z has none . Other Apple products driving down unit costs contributes to "cheaper for Apple" also. ).
