I had some stone walls under construction when I logged out. Over 24 hours later when I logged back in, my walls weren't done. It appears that the workers started on the next wall sections as soon as I logged in.
My guess is that the wall sections currently underway when I logged out were completed while I was offline, but then the workers didn't move on to the next sections until I logged back in. So when you're online they are smart enough to move on to the next wall section, but for some reason they forget to continue to the next section when you are offline. But then they remember when you log in again.