Scorch was talking about focal planes. Think of it like this, in second focal plane the reticle appears to be the same size, relative to your eye, no matter what power the scope is set on (the target grows larger,the reticle does not). Therefore the reticle has to shrink or grow with the power change.
On first focal plane scopes the reticle always stays the same relative to the target. This means that the reticle will appear to grow or shrink inside the scope, but it will always cover, ( or subtend ) the same amount of the target (the target and the reticle grow larger). To make the reticle do this differently it must put on different sides of the power adjustment system ( different focal plane ). That is why the BDC changes with the power setting. As stated before most ballistic drop compensation scopes are set to work with most cartridges at max power. This adjustment also allows you to "play" with the power settings and find which one works best with your gun.
And yes wild Bill i believe all Nikon BDC scopes are second focal plane. So yes they will all be different except the main cross hair. Yes your other scopes might have a small change in poi with power adjustment. Shoot a group at each power setting to check your scope. It should not be much if it is even noticeable.
