"With 83 burial chambers, all containing cremations, with the odd exception, compared to Cornwall's 7, there was thinking that the less important dead were shipped across from the mainland for burial. The discovery of the odd settlement below the high tide level hardly explains the high number of passage graves compared to Cornwall."

Perhaps it has something to do with the sea, the number of burials there, whether they came from Cornwall or not.
It could also be tribal, if all the passage graves were similar, we talk in terms of settlement but you could just as easily talk of a distinct native island people following the same patterns of customs and ritual...