Over the 20 year plus period the seals deteriorate and let a bit of excess air through.
If you live in a warmer climate this is exacerbated because warm leaks out when you don't want it.
The seals in my car are fine. When turning the central wheel to seal off the centre vents they are sealed up tight.

In my Euro W124 AC cool air is emitted from the centre vents whether or not the air circulation is closed to interior or fresh air.