unless your actuator is connected directly to the compressor housing, the actuator will normally 'see' less pressure than the compressor housing (or inlet manifold or wherever you are measuring boost).

it's not a case of the actuator does nothing until you hit x psi and then it opens the wategate to maintain that boost pressure - it's always seeing some boost pressure on it's diaphragm that is pushing against the spring inside it. as soon as that pressure starts to overcome the spring force, the wastegate will start to open slightly.

as your t34 has a larger turbine housing than your t3, it is easier for the exhaust gas to escape, and this will affect the boost level that you get.