These corners are symmetrical with the axis of the monument towards the Summer Solstice Sunrise and Winter Solstice Sunset - see http://www.sarsen.org/2013/06/triangulating-axis-of-stonehenge.html
As can been seen from the photo and the plan below the standing Stone 56, part of the Great Trilithon is twisted to this line. The Great Trilithon was on the midpoint, straddling the acknowledged axis but doing so at an angle.
Looking North West from behind Stone 10
Click any photo to embiggen.
For my theory on why the Great Trilithon was twisted see - http://www.sarsen.org/2012/07/stonehenge-resurrection-alignment.html and my leaflet