Snow, frost and some strange thawing

Winter has arrived at Little Hampden. Some wonderful frost this morning:

Monday’s snow had thawed in a rather strange way yesterday. We have a small square of concrete in the lawn – it’s where the clothes dryer plugs in. The snow above this had thawed completely:

Also the snow on the concrete parking area had virtually thawed but on the expansion joint between the two slabs, there was still snow. This joint has a small area of earth and moss – no more than one inch across:

I’ve been trying to work out why this has happened – but I’m having problems. If the ground – earth, grass and concrete – had all been cooled to about the same temperature before the snow fell – a reasonable assumption, I think – then the heat transfer to the snow would depend on the available heat in the material – concrete or earth. I don’t know the specific heat of the various materials but water is typically five to ten times larger than solids. This is why freezer bags – things one freezes then uses to keep things cold when there is no fridge – are filled with liquid rather than being solid. So the grass and earth in the lawn around the square, and the earth and moss between the concrete parking slabs would have more heat to give up to the snow, causing the snow above these to thaw more than that over the concrete. But the opposite actually happened. So what is the explanation?