The same skill level should yield much higher absolute numbers at a lower rank. There’s also a tendency in low rank for players to chip damage each other constantly with a combination of bad positioning and bad aim, enormously inflating ‘average’ damage and healing. It’s also possible and has been claimed that smurf performance distorts the PBSR’s understanding of average performance in very low ranks (Bronze/Silver).
It is true that mature accounts will tend to gain or lose smaller amounts of SR … there is a ‘confidence’ number based on how much you’ve played recently and how stable your ranking has been. That may also account for some differences that you see between accounts.
They never alluded to any such thing, they simply failed to specify either way. Consequently, players have verified experimentally that you start in the same place and can rank up (or down) equally easily regardless of QP performance in the level up phase. Heck, you could level up in Arcade and Custom Games and never have a QP MMR at all if you really wanted.