I'm reading a Citation M2's Pilot Training Manual, and it says:

...When the throttles are advanced for takeoff, the bleed air turns on (if not already on for heating), and the pressurization controller starts closing the outflow valves to pressurize the airplane. The cabin altitude may descend by as much as 200 feet during this pre-pressutization prior to lift off.

What's the purpose of the pre-pressurization prior to lift off?