Are you sure it's the highest lvl Eidolon of that element that gets used?

Here is why I ask. I just switched my primary Dark Team Eid to Apoc, who is only lvl 50. I have another one of my teams that is set as dark to upgrade the grid, and the Eid for that is a lvl 70 Medusa. One of my friends commented that they noticed I changed my Eidolon. So based off of that experience it doesn't seem to choose the largest lvl.