I had a very strange situation, too, and I dunno if it's the delphi program or if it's their data, but anyway...

I put in a route generated with the FromGM.exe program. The route was as accurate as expected (approx 100-300 feet off at any moment) until a certain point right in the middle of the route where it suddenly got off by (what seemed to be) a couple miles. I was still following the same path directionally, but my current-position icon was about 2 miles east of the path.

I checked to make sure it wasn't the GPS data by loading another route (an older one generated with parse_google.py) which used the same highway, and it was just fine.