I have lived in NYC and Austin and I grew up in southern Alabama. Austin is much hotter in the summer. It's nearly as humid as Mobile but tack on 10 degrees.

Average humidity during the summer: Austin: 90% Mobile: 100% Miami: 100% NYC: 54%

It's not even close.

Maybe its because people are outside more in NYC and don't have a car readily available with AC torqued to full blast? Or less buildings have AC / the subway is oppressive?