BTW, the glider's tail not only is gaining or losing effect when it's moved forward/aft, it's also gaining or losing amount and direction of lift. Horizontal tails work in "downwash" (and sometimes it can be "upwash", however the "up" and "down" can just be thought of + or - ) which can be more important than balance. anyway......

Moving the tail relative to the downwash generator obviously would change the effect of that wash. Full scale aircraft often have the benefit of stabilizer trim to deal with some of that mess. And it's not that simple, because the forces also change with different speeds, something that also happens with the balance of airplanes. Nothing is sound bite simple is it...

Keep in mind that the weights of (and in) our models don't change very much, while it's speed often has quite a bit of range. At slow speed the things on our models that keep it stable often don't work with very much power. That's one reason it's good to have the balance sorted out to be within the CG range for a model before it's maiden flight.