While I have no way to validate this, I highly suspect that the routing algorithm is also subtly manipulated. There is a route I drive with regular frequency that contains a roughly 20 mile section of two mostly parallel roads, one for through traffic and one for local. Every single time I drive through, Google routes me to the local traffic road. I know for certain the local road is slightly slower and it's also simply incorrect. The only way it makes sense is if it's a bug, or my hunch is that Google weights the route a little higher because it goes by a bunch of businesses that pay for advertising.