That could be possible Moss. The orthostat itself, although I didn't measure it, is about 5' 6" high so there is about another 18" of corballed stonework above it followed by the capstone and minimal soil above it. The 18" 'gap' could be holding back a reservoir of water behind it. Water does always seep down this stone in wet weather so no mystery there, it was just the dry conditions that puzzled me. See, after all these years there are still more things unexplained :-)