This plan is from Edgar Barclay's book of Stonehenge.
I noticed that Stone 10 is marked as being twisted out of a true circular position.
From the current EH guidebook, circle added.
From Cleal et al
Stone 11 is out of line because it is leaning outwards, but 10 has never been restored or moved so it seems to be in its original position. Examining it on the ground the twist is quite obvious, and I have spent quite a few hours chewing over the problem as to what it could mean. Could it be related to the idea of 11 being an entrance way?
This morning the solution came to me thanks to one of Adam Stanford's Aerial Cam photos.
The ground level may be twisted but the top with the two tenons is exactly right. What amazing engineering!
Click to enlarge any picture.