I could be wrong, but I believe that they do this on purpose for the first eggs.
Just as I was posting my above photo with the one egg, I saw mama & papa mating again. I think most birds are the similar - I'm willing to be corrected on that fact - but there should hopefully be another egg in the scrape in the next 24-48 hrs as a result of this afternoons 'activity' on the ledge.
The mother won't start roosting full time until they have a full clutch. They do this to try slow down the growth of the first laid eggs so that they all hopefully hatch within a day or two..... Rather than spread out over a week or two, which it can take to lay a full clutch.