Master builder or not I believe it was done, or in this case not done, in the name of efficiency. You don't see round joist in the living area even covered with lath, which brings up a secondary reason for square timber, uniformity in finishing plaster. I have noticed hewn surfaces on timber that was sawn, this was done to bring a fat area into plane. As mentioned before the cellar usually had poles for joist with a lime wash, brighten and assisting in preservation. The humble builder traveling the country side building barns most likely didn't have a calculator to check the statistics of his timber, something else was built in though. Same thing happens today.
Tim
