I don't think it's just between spline points, but I have found it tricky to work. It definitely has issues with mocrossings, and I've had issues with it reading across structures as well, such as solid (non-spline type) bridges. Sometimes it seems like the target scanner direction matters, other times not, as well. Also need to make sure switches are aligned properly.
I primarily use scanners now when I need to figure out gradients over a long distance. When it works, it's effective for this, but using it to measure mileposts would reguire measuring, moving a scanner, measuring again, moving it, measuring, moving, measuring, etc... until you find the exact point of a mile.