kml - Calculated distance doesn't match Google Earth

I have a google earth route that measures 190.1km in Google Earth.

I then pulled down the .kml file, extracted the points and measured them using Vincenty's formula (ellipsoid earth) and got 190.2.

BUT, if I measure the SAME points along the SAME route to a different distance along that route, I'm way off. Sometimes as much as 2km.

For example, If I want to know the lat and lon of the point closest to 100km, I measure the distance between all the points along the route until the measurement is as close to 100km as it's going to get. When I compare that point to the route in Google Earth, it's around 98km.

Any ideas of why this is the case?