electrical engineer here

if you’re familiar with memory allocations like in C or C++, you would know that before allocating memory on the heap, there is some memory used by the OS that is strictly inaccessible to the languages. taking that operation and abstracting it to the hardware level, before giving the rest of the unallocated memory to the OS, specific system processes and hardware reservations use some of that memory. When a resource monitor (like btop or htop) reports memory, it only reports memory within scope of the memory allocation inside the OS. that’s why you see 61.3 GB instead of 64 GB, ~2.7GB is being used to run the hardware.