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