I can't explain that. If I change mine from Fahrenheit to Celsius, the Nest Hub reflects the change almost immediately and shows the temperature in Celsius units, but does not show "C" or "F". On ours, Photo Frame is set to Fullscreen Clock.
Found the solution. Google Home account is set up on my husband's phone. I use it on my phone, but apparently can't change settings. The settings had to be corrected on husband's phone not mine.