Pick me Up
So, the dark fairy goddess said the reason he never rolled a high rarity unit is because in Niflheim they are all dead so he has to work with ascending lower rarity units, which save exceptions tend to have lower statuses at the same rarity as naturals (similar to something like FGO that growth and base stats have an impact but there are those like Saber Alter that have SSR-tier stats in spite of being an SR).
In spite of that he never managed to raise a 7*, so I'm gonna assume with what they are mentioning now, 7* units are either Masters that got isekai'd or units that managed to reach Earth somehow since the story keeps putting emphasis on the Interaction Force and how just the presence of an Observer from Earth already changes the fate of the timeline somewhat.
Since the MC got isekai'd as a 1* but it still partially a Master this effectively means that weird Interaction Force is in full effect and his growth breaks convention.