Why does Navigation keep choosing a different route? I have a route which I have taken a couple of times using Google Maps navigation and every time I go it takes me one of 3 different ways, even though I start and finish with the same location every time.

In web Google Maps the directions returns 3 results which are 35mins, 40mins and 41mins with varying routes, one is using 3 motorways, another is motorway-town-motorway and the last is through the "backroads".

Is GNavigation making this decision intelligently (traffic reports, time of day etc) or randomly picking me a route of the 3 available? This is in the UK.
Google Navigation uses traffic information in some countries, e.g. USA and UK. If you are in one of those, yep, it will pick an optimal route based on traffic.