In the GrainCloud prototype, it may be due to this EventExpression in the Amplitude parameter field:
0.25 * (1 - !Density) * !AmpLow
So the higher the value of !Density, the lower the value of (1 - !Density).
Could this explain the effect that you're hearing?
One way to test would be to replace the contents of the parameter field with something much simpler like:
and play the Sound again to hear if !Density and !AmpLow are independent of one another.