Primary Dimension Changes

Hi Brent, I'm getting a bit confused, I've been playing around with a new 29er hardtail design. It all seems to work fine until I want to change fork length to see the differences. For example I might have things in at an AtoC of 470(80mm sagged) and ST/HT angles of 73/71. If I then alter the fork AtoC to 490mm I would expect to see the drawing rotate around the rear axle. So the angles slacken approx 1 degree, the BB drop shrink and the ETT and Reach numbers alter to reflect this. What actually happens is the front end just elongates upwards.

Is there something I need to do differently or is this to do with the primary dimensions being set and only alterable manually?....thanks....Al