Maybe the gadget was at fault. This morning I was on a training ride, working at "event pace" (which for me meant trying to stay at 14-15 mph). I saw that the readout screen said Zone 4.0-4.3 a fair amount of the time, and my HR was hanging between about 138-144. That seemed weird, when only yesterday I barely hit Zone 4 at 162 bpm, and my legs were turning to rubber.
I couldn't remember my exact zones at the moment, but I looked them up when I got home. Sure enough, Zone 4 is 139-154 bpm. Zone 4 is considered "brisk". It felt like I was working hard, but I wasn't in distress. In fact, I felt pretty good.
My legs still could use some plyo, though. Then I could go faster with less effort!