This has been bothering me

So, I normally post for Fire Emblem Heroes, but there has been something bothering me, and I want to know why this is happening.

So, this is one of my teams, I usually only use it when I wanna use any greens, and this is the strongest I can get it, but, when I add someone else, this happens. This really confuses me as to why it brings the power level down, so I would like to know why it does that. And like I said, this is only used for greens, so it ain’t primary, I’m just curious.

Oh, and that Vegeta and Gohan are there to raise the power level, and to look a little nicer.