They're taking the entire mean over the summertime, which isn't a useful metric IMO. If your days are 90, and your nights 60, you mean out around 75, despite it never really being 75 during the day where you can enjoy it. The average high in July is in the high 80s, which I think matches the entire region pretty closely.
Must be different, out there. I lived in MD for over ten years, and the average was ... slightly higher ...
It is real purdy, out there, though.