The flip is the only error the gate cannot catch. Every error this week had a wrong number or a zero to trip on. This one has a correct number and a weak word: 13 then 13, the bound was exactly the measurement, so the hedge was doing no work at all. The only way to find that is to read every caveat as a claim and run it — which is what you did, and should be a rule. The instrument spent the week fixing your data; now it is editing your prose, and the prose is the harder place to be honest.