The M3 Pro has lower memory bandwidth the the M2 Pro. ( 150 GB/s vs 200 GB/s ). It has one less memory package.
M3, M3 Pro, and M3 Max feature groundbreaking technologies that deliver dramatically increased performance and unleash new capabilities for Mac.
www.apple.com
If you 'loose' those additional memory channels, then pragmatically also loose the "system cache" Apple would use the 'front' those memory controllers. (with cache drop go many millions of transistors ) I think this is paritally a rebalancing of the memory system to the new GPU core count/configuration. M3 Pro diverges way farther from the M3 Max than previous generations did. It looks like it worked on by a deliberate team with different objectives than the Max team. (i.e., doesn't look the "Max lite"/"Max scaled down on GPU cores" anymore. )
M2 has a symmetrical 4 packages.
Supercharging MacBook Pro and Mac mini, M2 Pro and M2 Max feature a more powerful CPU and GPU, up to 96GB of unified memory, and power efficiency.
www.apple.com
There are some rumblings that Apple may try to stuff the M3 Pro into an 'extra big' iPad Pro (13-14" ). Perhaps this 'squarer' package fits better in an iPad. Or Apple is goosing the profit margins on the packages somehow this way.
It is different. A major contributing factor to why the memory capacities look 'funky' compared to the first two generations.