Averages. U6 still has lots of Saiyans (who aren't planet pillagers in this U) and Frost didn't go around killing off strong races like Frieza did. So there's likely more relatively strong being in U6, even if U7 has superior S-class talent.
Also, I'm not sure it's been explicitly stated that the universe rankings have to do with brute strength. I have a suspicion it will be revealed that the rankings have to do with intelligence or prosperity or something.
Also, also>Dragon Ball and DBZ was consistent as fuck. There was never a plothole at all.