I was curious too!
But today I noticed the presets are back to 72ppi after updating the photoshop which makes me think it was just the software glitch on their end... but not sure...
I guess if you are a web designer, 72 is kind of a benchmark for Mac screen resolutions going back to the mid-1990s. 96 is the Windows standard of 640 x480 px.
It's kind of weird. Those two screen resolution standards are no longer common and I would think browsers today would adjust to fit the screen.
I wouldn't even worry about the number in the resolution field.