20 Meters is quite short, so assuming a certain speed, there might only be a few GPS probes for each pass, that might also look as if they were outliers.
While I work at a company that, among other things, builds a navigation system (with a twist of doing load balancing on streets; called NUNAV Navigation), I'm not really an expert in this topic. From working with GIS and GPS quite a bit, I know though, that a lot of things, while they are working very well, are not as exact as they look (e.g. bad GPS signal, especially when driving with low speed, map matching, etc.)