Of course they did. The recommended CG is determined by flight test, not theory. (At least in GP's case.)

The recommended CG with the tank empty is, in reality, the most forward recommended position. Balance the airplane there with the empty tank sitting in it's stock location. Then fill the tank and see where the airplane balances. That is the most rearward recommended CG. Take the tank out, put it in the center of the fuselage and then rebalance the airplane halfway between the recommended CG and the balance point you measured with the tank full. That is the recommended "average" CG and a good starting point for your setup.

Jim