I understand your point, the thing is, I suspect that many of us are on here because of an abiding interest in weather statistics, so when we see an anomaly we are interested. Like many of us here, I have lived in NC all of my life, growing up near Fayetteville. KFAY had traditionally been warmer than KRDU. Now though, KRDU regularly outdoes KFAY, especially on hot days. Last Friday on the day when I made my comments, KRDU beat KFAY by 5 degrees. So yeah, I admit I am skeptical.
It's actually a fascinating issue because UHI affects traditionally have a larger impact in elevating night temperatures compared to day temps, so why the "RDU bonus" seems larger during the day is surprising.
By the way, I lurk in the Mid-Atlantic forum a lot and they regularly had discussion about how unrepresentative the snow totals are from the DC airports because of microclimate. This past winter in their discussions someone made the point that the main purpose of airport weather observations is to give the pilots and airport officials weather data pertinent to their operations. They are NOT motivated by a love of meteorological statistics.
Lastly, the "RDU bonus" is something I can remember being noticed for some time now. I can remember Greg Fishel making a snide remark about it while he was still at WRAL. But it does seem to have become more obvious in recent years anecdotally.