This is an excellent example. In fact, it's a good classroom lesson a teacher would use to explain pitch stability.

By changing the location of the wing relative to the tail, the tail gains more effect or loses effect. It's primary function is pitch stability and it's ability to do that changes when it's leverage decreases. It's size doesn't change but it's leverage does. When it's got enough leverage it will hold the gliders pitch steady. The glider will fly level. It also happens that it balances the lift from the tail with the pitching moment generated by the wing, but that's another lesson. There happens to be a number of aerodynamic things going on that are significant to the pitch of models other than balance. All of them will be working at the same time. Weight distribution is one, but not all. And it's not the most important either. In fact, they're all important.

Humans seem compelled to think lots of complex things are far simpler than they are. Balancing an airplane, especially a model airplane, is one of those. A simple glider is a good example of how just one way to balance it really isn't doing just one thing. It's also a good example of why very, very few of our present model airplanes have wings that can be adjusted fore and aft. Those hand gliders that do often change their aero and weight balance when thrown and those friction held wings shift under load.

This model stuff really isn't sound bite simple.