Why does Yellowstone Park extend a tiny bit into Montana and Idaho?

Maybe the border of Wyoming was set up to follow the geography of the park, not the other way around. I mean, the park was there before the states were.

Basically it had nothing to do with the land near it. It is simply because that is where the areas of geothermal activity are. Most of the park is a giant caldera of a super volcano and has a huge geothermal ecosystem around it. The boundaries follow this ecosystem.

Much of Yellowstone’s boundaries are straight lines, not defined by geological features:
http://maps.google.com/maps?f=q&hl=en&geocode=&q=yellowstone&ie=UTF8&ll=44.492587,-110.297241&spn=1.865203,6.943359&z=8

These boundaries are man-made, as are Wyoming’s borders. I’m curious why they are parallel and very close to each other but not identical. Wyoming has like 95% of the park, except for these margins. Was this done for political reasons, to give each state a piece of the park? It does not seem like mere coincidence.

they partly follow the yellowstone river in, but the rest is just where it was surveyed and the geology changed types. I lived in Livingston growing up and visited the park often. When you drive there the borders are self explanatory for the most part. You can see the rivers, mountains etc that make up the boundaries.