PT6A-28 Rigging Question
 

For the guys who have experience rigging PT6's, what would you make of this...

PT6A-28's

As a test you smoothly bring back the left engine into Beta / Revers and the following occurs...

NP increases
NP will rise, but before peaking NG rises
Once NG rises NP drops off suddenly
NP will hold around 1200 RPM as NG rises

I know it seems like the Beta Range is too small, but the odd part is the NP drop with NG increase. What could cause the NP to drop rather then increase?

Not a mechanic, just a curious pilot.