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There are a lot of things that could cause this. It could have been built that way (it was 1880). It could be differential settlement.
The most likely reason is it was built that way. It could still be settlement issues that were addressed on the first floor.
Sag in the joists is much different than one end being higher than the other.
I would monitor it for a a couple months and make sure it's done moving.
You will need to have an engineer investigate exactly why it is moving.
It could be helical anchors to arrest the movement.