Propane tank question on Class C.

I recently took my Class C in to Camping World and had a drop leak test done on it for propane leaks. Just the normal check up before beginning the year. This is suppose to be the best leak detector test done on them since it is done under pressure. I also had them fill the tank with propane so I would be ready to go. It was a cold day temp wise when we had them filled (28 degrees). They filled the tanks till vapor started coming out the escape valve. Now after I am home and its warmed up a bit the gauge on the propane tank reads 3/4 full while the inside of the MH digital gauge reads Full. Why is that? Does it have something to do with the temprature the day I had it filled the reason it is only 3/4 full now? Just trying to get a good understanding of this and to also be assured that there isnt a leak somewhere. Figured the outside gauge would read Full now. Thanks for your responses.