If you write 511 in binary, it is 11111111. (8 digits) for 512, you write 100000000, which is 9 digits. The fmc is an 8-bits computer, which can not handle 9 digit binary numbers.
Apparently this is the case if you have a single fmc configuration in your 737. With a dual fmc, things might be different.
The 8 bits config was chosen because of cpu stability.