Perhaps there's a clue in the other circle (and surrounding structures) just up the road at Avebury?

well the Stonehenge area seems to have been auspicious since the year dot, viz the meso postholes.... but are those coincidental? Maybe not, the postholes adjacent to Bryn celli ddu have just been measured as predating it by 3000 years...

There's rather a lot going on with these places we don't understand isn't there?! O for the clarity of d.....ing rods.