[atlas] What is the meaning of 127.0.0.1 as probe resolver?

Hi, While looking at the result of built-in DNS measurements that use the probe resolvers, I noticed that a significant fraction of probes have "127.0.0.1" as a resolver. And the results are strange: performance is really bad, for instance measurement 30002 gives a median resolution time of 300 ms for these probes! What could be the meaning of this? It almost looks like a recursive resolver with no cache at all is running on the probes themselves. I was looking at this measurement: https://atlas.ripe.net/measurements/30002/ And here are some example probes that exhibit this "127.0.0.1" symptom: 6001, 6087, 6162, 6235, 6308. Any reasonable explanation is welcome! Thanks, Baptiste