Asymmetry

I've just installed replacement windows, and it seems like some of them are asymmetric. The house is 100 yrs old, so I don't expect perfection, but is this common? Attached are some photos. One is the top corner and the other the bottom corner. It seems the "finning" (?) fits tightly at the top and not at the bottom.

Pull the top sash down 1/4" and the bottom up 1/4".. Is there an even gap all the way across?

Well, there is an 8th-16th of an inch difference.

Have you had the installer out to look at your concerns?
... I generally like those gaps to be indiscernible to the eye, however, a home of your age can often presents some pretty major installation challenges. Regardless, the window needs to be installed too the point that it is operating and sealing properly.

+1

Is that a crack in the second picture?

No, there's no crack. It does look that way in the photo though.

With several windows, I can see light when I look down between the sashes. The installer placed some shims to correct this on a couple of windows. Someone on the forum mentioned adjusting the side expanders. What exactly does this mean, and are these just 2 ways to achieve the same effect?

Yep, the side expanders/ jamb jacks are basically adjustable shims built into a window at the meeting rail. The interior caulk will need to be cut and then they are adjusted easily from inside the jamb.