Routing curiosity

I have found a routing curiosity using MapQuest as source for the routing calculation as follows:

Cycling on a

highway=track

name=Römerweg

surface=concrete

tracktype=grade1

coming from the north there is suddenly a turn to right (west) to a

highway=path

surface=grass

trail_visibility=intermediate

width=1

which is followed by a track

access=forestry

highway=track

surface=grass

tracktype=grade3

width=2

and another track

highway=track

name=Dreimarksteinweg

tracktype=grade3

to reach the Römerweg again, instead simply following the first track (Römerweg) towards the endpoint.

I cannot image any reason why this happens.

BRouter presents the correct route omitting the detour in this case!

Does anybody understand, what happens here?

Answers appreciated.

Reinhard