Watching Justice League I got the feeling that the people at Warner Bros. looked at the Superman that was on TV's Supergirl and said "Why can't we have THAT Superman in our movie?"
I would be hard pressed to find any substantial difference in the way Superman acted in Justice League and the way Superman acted on Supergirl. Geoff Johns was one of the people that
created TV's Supergirl, and now Geoff Johns is the president of DC Films, so there is that connection.