It appears that there was a bug that was fixed about a decade ago that required GET FULL VARIABLE. I hadn’t noticed the use of FULL, which seems to be a workaround for the ancient bug.
I would assume that GET VARIABLE works like the AMI one. Generally they all call an internal, get variable helper, function, that actually handles functions as well as real variables and functions without parentheses, like EXTEN and HANGUPCAUSE.