why didnt my iPhone auto-switch times when I came to Phoenix

I live in Colorado and have my iPhone set to TimeZone Denver, but also have "Set Automatically" turned on in my iPhone --> General --> Date/time preferences.

When I few down to Phoenix on Friday, my iPhone and watch (and my Mac) all switched times from Denver to Phoenix (one hour later now).

But when we drove out of Phoenix to Wickenburg (60 miles to the North West), my phone and my watch both switched back to Denver time. Even though I was going further west. I ended up having to set my TimeZone to PHoenix and turning off automatic mode.

I tried rebooting the phone to see if something got "stuck", but that didn't help. The clock reverted to Denver time immately after rebooting.

Any thoughs as to why this would/could happen I've never seen this particular manifestation of the time zone messup before when I was traveling...

Thanks!