A system variable is not defined and stored as a proper variable the same way all other vars are. For example, CHKVAR will always return false (0) for a system variable—even a writeable one. In addition, accessing a system variable through functional VAR will throw Undefined variable.
Take the following prompt as an example (lines starting with > are DIRECT input, all others are output.)
This leads to the conclusion that system variables are specially compiled keywords and not variables which point to some static system value; even writeable ones probably compile to a special assignment instruction when writing. This also possibly confirms the suspicion that TRUE and FALSE are complied keywords which simply represent 1 and 0, instead of being true variables themselves, making their associated constants redundant.