Marvin,
You're on the right track.
The static sources outside the airplane are located and designed to minimize the effects of the slip stream and uncoordinated flight. They are located strategically to prevent false readings.
Air flowing around the cabin/cockpit area experiences a drop in pressure. This drop in pressure draws some air from inside the cabin, as the cabin isn't tight. Air pressure inside the airplane is slightly lower than ambient.
If you get a chance to fly an airplane like a Piper cherokee or a Bonanza, you can see the low pressure effect around the cabin by cracking the cabin door slightly open in flight. Put a chart or piece of paper near the opening at the difference in pressure will keep the paper sucked against the slightly opened space. Open the space just a little more, and the paper or chart will get sucked out.
The door doesn't need to be open to experience this drop in cabin pressure. Because the interior of the airplane isn't sealed from the exterior, the lower pressure that exists around the cabin is also experienced inside the cabin.