Assuming that it isn't some weird scope parallax issue or something, I'm guessing the difference is... YOU.
For whatever reason, you are just executing your shots better with the 6x sight picture. It might be that you are mentally executing the "aim small" part of the "aim small, shoot small" equation when you see the smaller target image at 6x than the much larger appearing target image at 18x.
You might try putting a small 1/2" or so dot in the center of your 6" target to use as an aiming point and see if that improves your groups when shooting at 18x...
