yeah the reason your boosting higher is because the actuater isn't allowing as much air through as it should so by the time its reached the actuater and the expansive properties of denser air have been accounted for it doesn't fit through the gap.
My turbo petrol used to do it due to the wastegate size caused by changing the air filter and exhaust system, it would go from a standard 6PSI to 11PSI then 14PSI on cold days, i just got an external wastegate