When you open the priority there are normally around 3 girls shown - mostly 5/6*

If you roll a 5* or 6* when randomly determining which 5 or 6* you get it is more likely to get one of the ones shown, not a guarantee, and it doesn't change the odds of rolling a 5 or 6* girl in the 1st place.

If you are in no rush to get new girls what most people do is hold them until a priority comes up with girls you like

Thanks for the replies, I understand now.

It looks like the JP version got a second evolution for some girls yesterday. It's probably a long way off for us but it's nice to have some new art.