I am aware of a DIY route that was submitted and agreed @ 203k. When the rider came to submit the same ride but in the opposite direction he was told it only measured 201k. Not a problem and both rides were done.
They were "circular" routes rather than out and backs measured on the same copy of AR. So does AR have something in it that treats the same route ridden anticlockwise from one ridden clockwise.
1-way systems and even large roundabouts will contribute to this effect.
This is why I think its rather unfortunate that the accumulated wisdom when using Google Maps for distance-checking is to switch it to 'Walking' mode - which gives slightly under-distance results.
Driving mode and 'avoid highways' and 'show alternatives' is the best way to use Google Maps IMO.
There is (or will be) a Cycling mode but that won't be much use to us - think of it as a Sustrans mode ...
And Google Maps is cross-platform, and I honestly feel is far superior to Autoroute in every way, I just wish the powers that be would make this the officially-sanctioned tool.