My thoughts are:
When you rev the engine the intercooler is pumped with air pressure.
Because you are in park the engine and supercharger rpm comes down very quickly, the supercharger quits pumping allowing the intercooler to discharge through both the supercharger and BOV.
When you are driving down the road and let off, the engine rpm stays up and the super charger continues to pump forcing all the air to discharge out the BOV.
You can temporarily move the boost gauge line to a point between the supercharger and intercooler to see if there are any spikes in pressure when you let off.