I don't think they pre-assembled the whole thing upside down ;)
Bugger, and I thought for a moment you'd sussed it there ;-)So, what are we saying if we're saying they're not cock-ups? That they're recycled stones from another circle (or circles) either at Stonehenge or further afield? It makes sense that the Stonehenge builders wouldn't want older mortise holes to be visible from the ground - that would look sloppy.