If you use one of the games' default weather settings, pressure will always remain constant. If you want variations depending on where you're flying use real world weather. That way, whenever you're under 18,000ft pressure will vary. Just remember that above 18,000ft your altimeter must be set to 29.92
As already stated, this is NOT your altitude. This is the barometric pressure registered at your location. It is given to you by ATC so that all planes flying in that area have the same altimeter settings.
Regarding the altitude question...It doesn't matter what the airport altitude is, if ATC tells you to climb to 9000ft you just set 9000 in your autopilot (make sure you have the correct barometric pressure) and flyaway. If you requested a higher flight level ATC will not clear you all the way to tht altitude....you will have to "step" climb waiting for atc clearance.