I think the clue was in your mention of a minus 6 degrees temperature
Air pressure drops as temperatures fall. So if you inflated your tyres to the correct 29 psi when the temperature was 10 or 12 degrees C, they would give a lower reading when the temperature drops.
As a general rule of thumb, I gather a drop of 1 to 2 psi for each 10 degrees C temperature drop is pretty standard.
So if you pumped them up to 29 psi on a warmish day, the pressure reading could well drop to 25 psi on a cold January day when it is minus 6C outside.