That's an excellent question. Latitude and longitude are straight in the projection he is most likely using, but great circle arcs are what we consider straight on the globe. The kicker is that most political boundaries are neither. They are Rhumb lines, aka loxodromes, which mean that someone started walking in a direction and kept going.

The standard solution for this is to put lots of little points into the state GIS definition, so that the points get transformed correctly. That way short line segments don't differ by more than a few meters. That means you have to watch out for simplified state representations, but not much else, unless you're being a stickler.