Remember Bernoulli? Lift is created on the wings because the air pressure decreases as the air moves faster. Same thing around the fuselage, but to a much lower extent. The air flowing around the fuselage creates a low pressure area so air in the cabin moves from inside the cabin to outside causing a lower pressure inside the cabin. Opening the cabin air vents can cause the pressure to be higher inside the cabin than outside.
Many general aviation airplanes that are used for IFR flying have two external ports that are placed well back along the fuselage. The ports are usually positioned in areas where the boundary layer of air tends to be very stable, which lessens the effect of the static ports being hampered by turbulent flow and the Bernoulli effect.
Barry Schiff has an interesting explanation of how the pitot-static system works and shows how the placement of the static ports affects the reading of the instruments.