First of all, the digits are Tony's rankings, not percentages. In fact, it looks like 82-83 percent of stations have reached 80F. But more importantly, this is a meaningless statistic for a number of reasons. It's affected by the average elevation, latitude, urban character, etc. of the station mix. If there's a higher percentage of stations in high latitudes and/or higher elevations, you would expect a lower percentage to have recorded an 80F day before the end of May [also, I doubt all of May 2024 has been included in the data -- the month is just now ending]. Further, it's affected by equipment changes. It's well established that a lot of the automated stations report somewhat lower maxima than traditional LiG thermometers housed in a shelter.
Even ignoring all this, it's not really meaningful. It looks like pretty much every year at least 80 percent of stations have reached 80F by May 31. There's only about 15 percent or so of stations that vary in a given year - many of which are probably in the same general area [somewhere in the northern US]. Many of these places probably don't see 80+ until late in the spring, so all it would take is a cool spell in May to keep them from reaching that temperature. Meanwhile, the rest of the country could be baking... and even the places that fail to reach 80F might have been way above normal for the majority of the year.