What exactly is the difference between the A-series and M-series of chips? It'd be a lot easier for Apple if they just had a single line of chips, right? I'm under the impression they have the same op-codes so... is it just a differing core-count? or do they run at different frequencies? Different amounts of cache/registers?
There is no fundamental difference in architecture. They use the same IP blocks (CPU, GPU, encoders, ANE). Just more or less of them. Roughly speaking you can use this rule of thumb (doesn't always work, but close enough) for CPU and GPU:
M is 2x A for GPU and big CPU cores, but not little CPU cores
M Pro is 2x M
M Max is 2x M Pro in terms of GPU, but not CPU
M Ultra is (literally) 2x M Max
The A Pro chips sit somewhere in between A and M, having more GPU cores and more system Cache.
A bit more in depth:
The architecture is the same, and maximum clock speeds also are the same. Thermal envelope, however, is very much constrained in mobile devices. But: An M4 in a MacBook Air also doesn't have the same thermal envelope as an M4 in a Mac Mini. So even same name chips aren't really Apples-to-Apples comparisons. And that's before factoring in that not all variants have all cores enabled (less GPU cores are common, the iPad Pro has an M4 with only 3 "big" cores, etc) due to binning.
Also do different chips have different memory busses. The same "double with each tier" metric *roughly* applies here as well. So bigger chips have wider memory, which means they can access more memory in less time, which is mostly interesting for GPU and ML.
Adding to that different chips may have different storage controllers, differing amount of Thunderbolt / USB IO, and display controllers.
So, TLDR: Apple is using the same IP blocks in a generation, yes, but they assemble different configurations with them. This is common industry practice.