In an unpressurized cabin, why is the cockpit ambient pressure lower than the outside pressure?
The air moves around the fuselage, which causes a localized increase in velocity around the aircraft.  This increase in velocity causes a slight decrease in the outside air pressure, which effectively sucks air out of the fuselage, making the cabin at a slightly higher pressure altitude. 

The effect is localized.

While the reduction in pressure, and increase in pressure altitude inside the cabin is small, many GA aircraft display a 40 foot or so difference in pressure altitude between the alternate static port (inside the aircraft) and the pressure altitude with the factory static port.