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It's only 19% 1 out of 21 blocks when the backup witness signs a block.

Or maybe it's 19% whenever the most recent backup witness signals anything else but 20%. Something like that. There are still 10 witnesses at 20%.

What? Again?

Oh sorry, it's back up to 20%! I just rechecked. Interesting how this happened.

I think people are reshuffling their Witness votes.