Low-level software is responsible for most memory issues, but there are definitely electrical reasons, too.
For example, I have a devboard that only support 1 GiB of DDR2 RAM, but it's a 64-bit system and the memory controller on the CPU was supposed to support at least 2 GiB of RAM. Meanwhile, another board that uses the identical chip runs 2 GiB RAM without problems.
The engineers of the devboard briefly explained that the problem was electrical. The memory controller itself has inadequate drive strength, adding more RAMs would increase the load on the DDR bus and destabilize the system. On the other hand, the other board had better PCB layout so the problem did not occur.
I have a Mini-ITX board, I noticed that if I activate XMP, the board will cease to work, but it works with 4 GiB of RAM at standard frequency.
So it seems memory is a general problem among Mini-ITX boards? Perhaps the reason is that these boards have less available space for routing, fewer layers, and targets a lower price, so they trend to have worse electrical characteristics?
Quality of the PCB and the number of layers definitely plays a factor I'm sure it isn't limited to ITX though. I have noticed compatible differences between super robust Intel ITX boards vs ECS thin and wobbly ITX boards, with the ECS board having more issues, quirks and what not.
I had something similar with my 2011 MBP. I had it running just fine with 16GB of ram even though it officially only supported 8GB. After having the main board replaced because of a defective graphics card I could not run it with more than 8GB without it crashing continuously.
For example, I have a devboard that only support 1 GiB of DDR2 RAM, but it's a 64-bit system and the memory controller on the CPU was supposed to support at least 2 GiB of RAM. Meanwhile, another board that uses the identical chip runs 2 GiB RAM without problems.
The engineers of the devboard briefly explained that the problem was electrical. The memory controller itself has inadequate drive strength, adding more RAMs would increase the load on the DDR bus and destabilize the system. On the other hand, the other board had better PCB layout so the problem did not occur.