Ummm, there is certain logic to it.
Switch rotates through "Off" "R" "L" "BOTH".
When testing "L" the right magneto is earthed and vice versa. Otherwise the sequence would be Off L R Both.
Of course if I was standing facing the propeller, which turns clockwise seen from behind, then it would appear I was turning the key anticlockwise because I was looking at it from ahead - as a propeller puller. Now if I was in an aircraft with a hand swung pusher propeller it would be completely different wouldn't it?