Or it could be the mid-level temps were the same, but surface temps were actually cooler...but I don't think temps were cooler than average over the entire world
Thinking about it, I think our instability "averages" are too high. If data only goes back to 2000, and we've been in the upside cycle of hurricanes, of course the "averages" will be too high, because it does not incorporate the downside cycle. It has been over a decade since we entered the "up" cycle, perhaps we are starting to swing back into the "down" cycle?
If the times from 2000 - 2010, the vertical instability was "100", and that is the only time period we have, then the "average" is "100". So of course getting a reading of "20" is going to look extremely odd. But if the 100 year average is "50", then that "20" doesn't look so odd in the long run.
Cooler surface temperatures being a factor is highly unlikely. Since we're talking about a basin comprised almost entirely of water, the low level temperature anomalies will invariably mimic SSTAs, which were largely positive.