When the fastest route is not the fastest route

The above screenshot is from my iPhone yesterday when we got caught in traffic. My wife was driving and I wanted to check that what TomTom was telling her was correct.

And above is a screenshot from my wife's iPhone at exactly the same time. We have the exact same version of TomTom installed, we both have 'fastest route' as the default setting yet we are asked to go in different directions.

I refreshed mine a few times and did the same with hers, and we got the same results each time. Is TomTom choosing which way we go to aggregate the traffic flow over more than one route? If so that is not only clever, it is a little disconcerting to me.