Its possibly to do with the Signal Strength thats being seen by the PicoScope and Pico Diagnostics Software.
Unfortunalty i have seen this quite a lot on GM's i.e. Vectras, Astras etc. When scoping the Alternator Output on PicoScope its quite 'unstable' compared to other OEM's Output.
Have you tried really loading the alternator up to get a better 'Alternator Ripple Signal', for me i have to have the following on then i find it becomes more stable and gives a better reading:-
Interior Heater (Full)
Heated Rear Screen
Give this a try and see how you get on