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the reason they fell back from 42% to now 39% is because there's been a few miners pulling out for the reason of protecting the bitcoin network.
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I haven't yet checked, but at a guess I'd say that could easily be due to variance. A larger drop over a longer time period might be indicative though.
what about that ever growning 'unknown'? That could work with ghash to do the 51% deed correct?
why is so much hashing listed as unknown anyway?