The comments tried to explain, but it probably wasn't too clear... in rare cases, some artist searches return multiple results from MusicBrainz. We've only experienced this once, with the band Bastille. Bastille returns about 4 results. I tested what came back from Setlistfm from the other artists named Bastille (passing in their IDs) and only Bastille, the popular band, returned anything; everything else came back as "not found". So I figured I could quickly run each result against the response from Setlistfm, returning only the one that doesn't result in not found.