I have also found that since moving to OWM.
The high temperatures for Sweden is completely off the mark most of the time. When I look in the OpenSprinkler app, it looks more like the average temperature than the high.
During nice summer days here we often have 15C during the night and 25C during the day, but typically see 15C as the low and 17C as the high.
I have looked at other areas in the vicinity, but it makes no difference.
To some extent this can probably be blamed on OWM, but it might not be the whole story.
I wrote to OWM here, but did not get much of a response: