It could be the remnants of a stone cairn that's had the overlying cairn robbed for building stone. Having a look at the middle might help - particularly if there's something there, such as a cist burial, a wrought iron ring attached to a stone flag or a couple of buried bottles of ginger beer. The Mare and Foal stone circle, in Northumberland, was reckoned to be once cobbled. (It also has an outlier that Hob has never bothered to find!)