Aerodynamic Centre question
 

Can anyone help me with this (probably stupid) question?

I've been reading AP3456 and it says that 'typically' centre of pressure moves from 30% to around 20% at the stall. It then says that 'typically' the aerodynamic centre is around 25% chord, though for wings with reflex curvature this can be up to 23%.

The thing I can't get my head around is that if you're taking moments about the 25% point, then when the centre of pressure is aft of that point, the moment coefficient is negative. When you take moments with the centre of pressure forward of that point surely you're going to have a positive moment coefficient. Yet by definition, the moment coefficient should be constant.

What's the obvious thing that I'm missing?

You're not missing anything. The simple theory, where the term a.c. was derived, only works in the linear range of CL vs alpha.