Yes, this is probably it. More specifically, any 128MB areas in the physical address space that do not have "System RAM" will not get a memoryN directory. You can view all the "System RAM" areas in /proc/iomem (if root, otherwise they're all 0's).

The most relevant kernel code is here:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

I am one of the original authors listed at the top of that file.