M2 pro has 256 bit wide memory, mostly benefiting the GPU perf.

M3 pro has 192 bit wide memory, GPU improvements mostly offset the decrease in memory bandwidth. This leads to memory options like 96GB.

M4 pro has 256 bit wide memory, thus the factor of 2 memory options.

The 96GB option was with the M2 Max and M3 Max chips, not the M2 Pro or M3 Pro.

DRAM chips don't just come in power of two sizes anymore. You can even buy 24GB DDR5 DIMMs.

It is interesting they only support 64gb and then jump to 128gb. It seems like a money play since it's $1,000 to upgrade for 128, and if you're running something that needs more than 64 (like LLMs?) you kind of have no choice.