I think you're probably talking about location and aspect Darren. I was trying to take some pics today to illustrate the point but didn't do well. On the north side of the house it was -2 degrees and the frost never lifted all day. On the south side the sun was shining and the thermometer on the wall was at 20 degrees.Much warmer than the greenhouse. I had plants standing out there (didn't get good pics as the sun was too bright) basking in it. I think this is what is going on with your greenhouse.
Above is the frost on the north side BUT
these plants are in the sun
The temperature advantage probably carries on overnight. Have you checked the day time temps in the same locations.