One of the new things you find in modern car is real time tire pressure monitors. I've learned that in as little as a mile of driving the air pressure in the tires on my Chevy Volt, can increase by 3-4 psi! That's just neighborhood driving at 25 mph, not at highway speeds.

So, when VW says to inflate your tires to 40 (or 48) psi, they mean cold, before you've been driving on them. If you check later you are probably putting too little air in your tires.