Let's see if I can explain it to everyone's satisfaction.
On page 41 of the US 997 Turbo brochure it listed what the ecu in the 997 Turbo measures, of note, it measures
-pressure upstream from throttle;
-temperature of airflow upstream from throttle;
-temperature of ambient air;
-ambient air pressure
Remember the throttle body is aft of the turbocharger/intercooler and is part of the pressurized track.
That first item must be where the boost bar takes its measurements.
From the chart it seems like the ecu mappings all work off pressure, not air mass. I imagine the ecu has a absolute, not relative, pressure target for the intake plenum and it bleeds off the excess pressure using the bypass valve which it also controls.
In cold weather/denser air situation, the ecu knows the lower air temperature and higher ambient air pressure and to hit the target absolute pressure mark, a lower pressure is needed, hence the lower boost value we see on display. In other words the boost bar is a relative pressure, not absolute pressure.
TB993TT is correct in his theory, just not the terms.
NA cars are quite sensitive to change in air density/air temperature as there is no way for the engine to compensate for loss of air pressure, hence the large change in performance in varying air temperature/altitude. Turbocharged and supercharged cars on the other hand are relatively unaffected as the ecu works off a fixed target, the turbo/supercharger just have to work harder, but only up to the max ability of the turbo/supercharger. There will be a point where even the turbo/supercharger simply cannot keep up with the ecu target and that's when the performance of the car drops off.
In theory a ecu can be programmed to use relative pressure as target and the turbocharger/supercharger programmed to produced a fixed boost value. A car like that would take advantage of colder denser air to produce more power but it would produce less in the hot weather, but such a car would have wildly difference performance for different markets, ie the same car will be much slower in Dubai than in Finland and THAT won't make the oil sheikhs happy