The Italians have it right. The pump pressure is being measured at no flow -- pure static pressure. But the pump curve on vibratory pumps plus the OPVs they usually use falls off much faster with flow than that of rotary pumps. This means that if you set the static pressure of a rotary pump at 9 bar, it will flow at roughly that rate. Vibe pumps need to be set at roughly 11 bar for their flow to be at 9 bar at espresso rates. Some of this also depends on the properties of the OPV. Ideally you would use a Scace II to set the pressure at esresso flow rates.
Many people prefer espresso brewed at slightly lower pressures, translating to initial settings of around 9 bar for vibe pumps and 8 bar for rotaries.
Finally, the rate at which the pumps rise to full pressure is far faster for rotaries than vibes. There are some Italians who believe vibe pumps set at 11 simply make better shots than rotaries set at 9, but that you can't turn up the rotaries since that would destroy the puck in the first few seconds of the shot.
As you can see, this is mostly tuning and engineering lore, some of it contradictory, rather than results backed up by well designed blind taste testing. If you are able to adjust pressures, you are best off using your own palate to decide what you like best. In my experience, pressure adjustments made a difference on some machines I've owned or worked with, but not on others.