It was presumably the upper threshold before air conditioning came on.

Explanation for those wondering why this needed explanation: Most thermostats set the lower threshold before heating comes on, so turning it up would use more energy.

thanks to you and everyone for the explanation - I haven't lived in the U.S for 20+ years now so I forgot all about air conditioning.

Raising the temperature during a heat wave is the same as increasing the threshold where the ac unit kicks in. So instead of all these ac units clicking on to cool off houses at any temp above 70 degrees, the ac units click on at 80 degrees. That change saves a ton of energy.

One of the things I do during the year is set the AC according to the outside temperature. During the winter months, my house is set to 70 degrees. As the average outside temperature rises, I increase it by 2 degrees for every 10 from 60. So when it's 100 degrees outside, it's 78 indoors. I have an older unit and watched in horror last year as it struggled and failed to cope with 110 degree temperatures.... I basically just gave up, shut it down, and waited until night time to turn it back on.

Raising the temperature the thermostat is set to maintain the house at, thus less power is used to cool it in a hot environment.

because air conditioning runs less.