It's possible that the uprights may have been planted with the tenons pre-cut, then measurements taken and the mortices cut in the lintel to suit.
2 things. 1/ Some of the lintels have holes on both sides so they screwed up at least once. 2/ The stumpy stone is too small to have had a lintel so there must have been a gap in the SW. therefore there was never a complete circle of lintels. PeteG