Very easy to explain that.
Think of it this way. If I am not swinging "on plane", that can either be over the top or underneath plane... as I approach the ball, I have to not only time the face of the club but Also the path of the club.
If I am approaching the ball from underneath plane (from the inside) and my club face is square to plane, it is WIDE open at the golf ball, so at the instant it reaches the ball I have to snap the face shut to get it square to target, which would be closed to plane.
If I approach the ball directly down the target line and "on plane" I now ONLY have to worry about that club face.
As far as "one plane" "two plane"... One plane doesnt even really exist, there are "flatter" and "more upright" but a TRUE one plane swing would have you swinging the club around your waist, ie- ON shaft plane. But because of the design of a golf club (the fact the shaft comes out on an angle) we are required to make plane shifts throughout the swing.
But that goes back to my original note... if we agree that there is a plane that is too flat, and one that is too upright, then somewhere between those two has to be optimum, for you.