GPA / 2 states - frustration x100

I’m confused. If his unweighted grades are 3.1 and 2.89, how did he end up with a 2.55 GPA?

Our district used to not combine grades, but people started transferring from other schools and would take their harder courses there and their easier courses at D20’s school for a GPA bump. District caught on, and now they do combine.