The solenoid most likely opens slower because more time is needed to buld up the magnetic field in the coil. The field build up is not linear. It starts building fast but it tries to approach a maximum value (an asymptote). If the field strength necessary to pull against the pressure is close to the maximum value, it takes a while for it to get there. Sorry, I'd need to pull out my math books to explain it better. It is also possible that the rubber seal compressing into the cone around the orifice affects the timing when it is close the the maximum pressure.
Stephen Morgret