The difference between these two chips is the number of cores. However, there has been new evidence that suggests the A12Z Bionic chip is simply a retitled version of the chip (A12X) along with a latent core enabled GPU. So A12Z has GPU (8-core), and A12X has GPU (7-core).
As per teardown site, TechInsights and NotebookCheck A12X contains one core and eight cores and displayed by the Apple. So, the fact is Apple did not use eight-core earlier, so A12Z is not the latest design. AnandTech also suspects that A12Z is a variant of the A12X re-binned.
AnandTech says, “There may be many speculative reasons for Apple choosing to do this.” “It is a common practice for the chip makers to disable the enable these cores in the next upgrades and physical cores.“
AnandTech also has an opinion that Apple may have done this deliberately. It would have saved them from developing and building a new core for the A12Z chip. It’s easy for them to enable an already existing core than to create a new chip with an extra core.