Apple watch always displays the wrong high temperature
I've found that this happens because of the delay introduced by the network and retrieving the temperature for your location. 

Many things have to happen to show you the temperatures. I see this same thing on my iPhone but I think it's more noticeable on the Watch because you're staring at it, waiting for the data.   You can be looking at the old data while the app gets the latest updates and then the numbers will change as the new data rolls in. 

In addition, some apps, such as Weather Underground, take an extremely long time to do their updates. I've even given-up using some weather apps because of this delay.