It appears that half of the memory is soldered to the board. That was also the case on my laptop mentioned above. It came with 4GB soldered, and 4GB in a slot. Why? I don't know. I was able to swap the 4GB with an 8GB and somehow it's still running in Dual Channel mode per CPU-Z so I guess it's no big deal, but I also find it odd.

Probably the model built around the lower-specced CPU has a "lesser" motherboard + power supply, that might draw less power on idle (so, better battery life) and be smaller/ligher, but would render the motherboard unable to output the spike currents required for the higher-specced CPU, nor the sustained current required for DRAM refresh of that much memory.

The model with the better CPU necessarily has a more power-hungry motherboard feeding it, and so can also feed current to 32GB of DRAM; but this likely comes at the expense of on-idle battery life.