I suppose the simple answer to that is the COP hasn't suffered a hard failure in the sense of open or completely shorted out. I assume that either of these cases would surely set a misfire code. The COP is actually failing in as soft failure mode with one or two turns of the coil shorted out, letting it continue to work except under high engine loads when it's output falls off and the intermittent misfire occurs. that is why a stress test is used to illustrate the weak COP.