That's likely a side effect of SU type carb in that the slide is vacuum operated which also controls fuelling, drop in vacuum and carb slide should go down, even though traditional air leak on engine side ordinarily results in increasing revs.
So conflicting outcome based on equipment peculiarities. The carb damping oil will stop flutter and control rate of change, but will ultimately settle to where vacuum dictates.
I used oil viscosity here to trim fuel "mapping" profile in setting them up, the SU type that is.