@Harvy66:
If you have an integrated video card, I wonder if it's allocating some of the memory.
Video is allocated 9 MB of system RAM. Still another 24 MB allocated elsewhere.
What is magical about 512 MB and OP cache?
This is a Virtual Box with 512 MB, and 9 MB video. However a physical machine with 2048 MB also has 33 MB allocated that pfSense does not detect. So if it only had 512 MB it too would not enable OP cache.
Just out of curiosity I doubled the display memory to 18 MB to see if that has an impact on memory available to pfSense. It did not. So I think Virtual Box system memory and display memory are allocated from host memory independently. In other words the display memory is not deducted from the allocated system memory.