A fine guess would be rather than folk from elsewhere donating stones to the Ring of Brodgar it was a case of their stone circles being cannabilised for it from force majeur. Also there are strong hints that some small stone circles may have become buried under later features.

i don't know what the relative ages are but it almost seems inconceivable that the sanctuary / avebury / silbury complex would not be an analogue of the stenness / brodgar / maes howe complex when the avenues were intact? that would make silbury a symbolic representation of a chambered tomb - like belas knap is a symbolic court barrow