just because a turbo can flow 15psi @ 3500rpm, does NOT mean it will continue to flow is at 6000rpm
consider this:
You are trying to blow up a balloon, but this balloon has a hole in it. YOU represent the turbo, and the hole represents the engine post-manifold. The size of the balloon represents manifold pressure.
With a small hole in the balloon (low RPM = low volume of air into cylinders) you don't have any troubles keeping the balloon inflated to the same size (boost level)
As you increase the size of that hole (increase rpm = increase air flow into cylinders), you find it harder to keep the balloon at a constant inflation. This is going out of YOUR EFFICIENCY RANGE.
A turbo is merely forcing more air into the manifold to increase the air pressure inside it. The engine itself is RELEASING this build up of pressure. As the RPM rises, the rate at which this air is "leaking" grows, therefore the turbo must push in MORE air to maintain the pressure. A smaller turbo will have trouble doing this at higher RPM.
Just because an RB25 turbo went spaz and hit 20psi @ 4000rpm or something, does not mean it would have kept it there until 7000rpm (on the assumption it didn't detonate immediately)