The whole series naming is confusing. ARM makes Cortex-A chips, like A53, A72. Cortex-M chips, like M55. So naturally Apple calls their chips A[number], and M[number], which don't correspond to Arm's chip numbers at all, but look just like them, and do have name collisions.
Both Arm and Apple offer A5 and A7 chips, but mean entirely different things. Apple's A5 is based on ARM's A9. Apple's A7 is ARMv8, but is Apple's first custom design, so not based on an ARM A chip. It also featured M7 motion co-processor, which is about to get its own naming conflict.
Since ARM also offers a Cortex-R and now a Cortex-X set of chips, it's only natural that Apple's next chips are either R or X in order to continue the confusion.