Sometimes it's not about a certain pro being good at a certain agent. Talking specifically for teams like GE and KC which had a complete new roster startup.
At times it could be that a certain player is not as good of a player in his role, but you're better at that role.
For eg, imagine player A is an S tier Jett and an S tier Kayo/Skye
Player B is an A tier Jett and an B tier Kayo/Skye.
The skill disparity from the Jetts is just one tier, (S -> A) but the flex one is of 2 tiers (S -> A -> B)
So overall it's better to put player B on Jett and Player A on the flex (depending on map)
In real terms, it's more like texture maybe a better Jett than SkRossi but because SkRossi is not as good as a Sova / Breach / Skye, Texture will play the flex and SkRossi goes Jett.