I think you will find that the "stall stall" is in fact occurring at the correct AoA.
My understanding is that the aural warning is generated by the FWS based on real AoA data, whereas the Vsw is based on Mach data.
At low levels the difference between the 2 is negligble, but at high levels the increased Mach causes a difference between the 2, which is enough to show the variation you describe.
So, in summary, I believe the "stall, stall" aural warning is indeed occurring at the correct angle of attack and providing you with the design margins.
Anyone else care to comment ?