The routing can do strange things if you want to, say, go up a side road for a cafe stop then retrace your steps back to the main route. Here is an example - I was trying to plot a GPS route for an organised ride this coming weekend.
Here is the "direct" route for a small section - the points are on the route being used into & out of the village.
If I try to add in the location of where the organisers have kindly laid on refreshments, at point 1, I get
Instead of taking me straight back to the original route it takes me some way further on, then onto a footpath to another road, does a u-turn there, back along the footpath before finally putting me back on the road and past where I just stopped.
I've noticed previously that trying to add in a small detour the route will sometimes take me up to a mile further up the road before doing the turn-around. (It is usually at this sort of point I just use the cycle.travel route for reference & manually replot it on bikehike.co.uk to get a usable GPS file :-S)