do not take self advertised bandwidth into account
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It might result a long-term up and down of relay bw. Imageing a case, where for some reason a relay reports a lower traffic. It will get a lower weigth which yields into lower traffic and so on.
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It invites people to manipulate the weight of their relays.
BTW: In IRC there are hints and tipps flying around how and where to change the Tor source code to manipulate the transferred traffic.