[thrdbndndn]

Makes sense.

When you said geography features, I was more thinking of natural barrier (like rivers, mountains): they were very crucial in shaping the old country border, but should have less impact today if both sides are already in the same country. So it's interesting that some of political division are still present around them, which makes me think history has its (unproportional) influence than the geography itself. But I guess it's always multi-factor and hard to tell in vacuum.

It doesn't help that in most of examples in OP, the major difference between two (old) countries is the economic one, so a certain degree of Matthew effect is there.

[t-3]

It's been true throughout history and is still pretty true that most people never leave the area they were born in. This would cause geography, history, and internal politics to overlap in somewhat non-obvious ways.