plus,  if the day you checked them in the Fall was  'high pressure' barometrically wise,  and today was a low pressure day comparatively..  you can see a little uptick too.     31 to 29  in/hg   is  about 1PSI.  Tire gauges  only measure the difference between the inside of the tire and  current atmosphere on the outside.
Air pressure changes about 1 psi per  10 degrees increase.   


Count your blessings...  I usually find tires  about half flat when sitting for 6 months.