The gaps in the trim at the top of the stairs (where the landing is) and the gap between the wall stringer and the wall trim lead me to believe the bottom of the stairs settled with the foundation at some point. It also may have shifted "forward" toward the bottom of the stairs, if that makes sense.
I've checked a few treads and they appear decently level, but it's possible they were leveled after the settlement.
Ultimately, my goal is to tighten everything up: remove the screws from treads (put there by ignorant owners) and eliminate loose treads/squeaking so the stairs are not so noisy. I also want to close or fill in the gaps in the trim as they are an eyesore.