Maybe it goes places that bikes can't go in walking mode, such as one way streets, pedestriance areas etc.
Nope. I checked over the whole route and it is fine.
I do two short cuts on my route that take off about 1km in Hoddesdon. First bit is cutting over from Essex Road (by the big roundabout) into Stortford Road. The second is going down Rye Road which is doable by bicycle but blocked off to cars by barriers by the water works.
However, this puts me on the more wiggly road up through the Hadhams, Pelhams, Meesden, Langley, Chrishall rather than the straighter B158/B1368 (which isn't the nicest road to cycle up).
Anyway, these ramblings were to point out several things:-
1) There's some leeway and common sense to be applied to routes.
2) Not all mapping tools are equal.
3) Rider honesty should come into it. If I planned a DIY 200 and only did 195km I'd hope I'd be honest enough not to submit the card for validation.