Not backward, but certainly side-to-side and up and down. The Jones Counter measured line is a lot straighter than the line traveled by the runner-worn GPS. That's got a series of small zigzags all along the way, all of which make the GPS traveled path a bit longer. How much difference these zigzags make is unknown to me.
How to duplicate the path of a runner-worn GPS in a test is quite a problem. Certainly the bike comparisons show the GPS to have, in many cases, good accuracy - but the bike tests don't replicate the path that a runner-worn GPS takes.