Good to know. I guess I will not shutdown underused relay just yet. A prepaid traffic allotment would be a terrible thing to waste! :-)
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A fine research question. A lot of the questions from https://blog.torproject.org/blog/lifecycle-of-a-new-relay apply here too.
One theory is that it's a result of variance -- they're both relatively small relays, so one stream that uses lots of bandwidth and randomly happens to pick this relay could be enough to skew a few bandwidth tests, pushing you down for a while.
In theory it should recover, since both relays are publishing similar underlying self-measured bandwidths: https://exonerator.torproject.org/serverdesc?desc-id=825feea5d643a228e121d0f... https://exonerator.torproject.org/serverdesc?desc-id=ec726fe88dbc60d18b3402f...
But it might take quite some time to adjust, since the bandwidth authorities don't measure very often.
--Roger