As alluded to, if you're using a small air nailer compressor to fill your tires, it may show 120 psi on the gage, but it won't put it into your tire unless it's running. When you first set the chuck on the stem, you get a little air until the tire and tank equalize, which is most likely higher than the point the compressor will start again. To overcome this, open the dump valve on the compressor until it starts and then stick the chuck on the stem. You're fooling the compressor into "thinking" the tire is now part of the tank and the compressor will try to fill the volume. You will have to repeat this for each tire since the compressor will shut off at its preset limit when you remove the chuck.
I used this method when I filled my tires with my 3-gallon Makita air nailer compressor. I now have a 12V Viair 400C compressor that runs to 150 psi and topping off the tires is no problem. Just did it yesterday.