Yeah its a very poor visualization. Its difficult to tell if it is showing the hottest wet bulb temperature of an area over the time period, or the most frequent, or what.
It is (likely) showing the 0.1% percentile of measured wet-bulb temperatures. i.e. in a given year, we expect 0.1% of the wet-bulbs to exceed this temperature or approximately 8 to 9 hours in a year.