I wonder why

Tier5/35
There was a little more snow last night but nothing very much compared with the east of the county. It was -4c as I got in my car to go to work though. Brrrrr. 
Much of the outside area is herringbone paving and it piqued my interest to see that the snow had only settled on the joins rather than the brick. I can only assume that the join holds water and freezes, thus keeping the snow longer. That’s only a guess though.