I've often wondered if the joint was craftily chosen - to allow a bit of lateral "give" if (for instance) some additional weighting was placed on or against the lintols.
In addition, rounded/tapered stub tenons might make it a lot easier to slot the whole ring together (allowing a degree of jiggling of the supporting uprights until everything fitted) - quite a consideration when you're trying to assemble a ring weighing scores of tons high in the air! After all, it might form a nice neat ring when laid on the ground but become a bit of a problem when you tried to erect it. It's the MFI flatpack syndrome....
I might be wrong about that but I haven't seen a queue of experimental archaeologists anxiously waiting to replicate that bit - even six inches off the ground!
Yes flexing would certainly be possible Nige to a degree, good point. A 'proper' mortice if flexed would either snap off the side under pressure or snap the tenon off.
Correct me if I'm wrong here (and it'll save me running through the DVD of it) but in 'The Secrets of Stonehenge', didn't MPP say that one of the lintols had the 'mortise' cut on the wrong side so it had to be correcly cut out on the other side? How can one prove that as it may have been intentional and meant to hold something else in place above it?