Bing Maps Directions Manager appears to be taking traffic conditions into account when generating a route

I'm having an issue where I'm using the Bing Maps V8 controls and I'm getting different routes on a map that appears to be related to the traffic conditions on the route. I have setup the DirectionsManager to set the route optimization to shortestTime which should says "The route is calculated to minimize the time. Traffic information is not used." in the documentation.

var map = new Microsoft.Maps.Map(document.getElementById('myMap'), {
    /* No need to set credentials if already passed in URL */
    center: new Microsoft.Maps.Location(-25.746000, 28.187100),
    zoom: 12
});

Microsoft.Maps.loadModule('Microsoft.Maps.Directions', function () {
    var directionsManager = new Microsoft.Maps.Directions.DirectionsManager(map);

    // Set Route Mode to driving
    directionsManager.setRequestOptions({ 
		maxRoutes: 1,
        routeDraggable: false,
        distanceUnit: Microsoft.Maps.Directions.DistanceUnit.km,
        routeMode: Microsoft.Maps.Directions.RouteMode.driving,
        routeOptimization: Microsoft.Maps.Directions.RouteOptimization.shortestTime,
        routeAvoidance: Microsoft.Maps.Directions.RouteAvoidance.none 
	});

    var waypoint1 = new Microsoft.Maps.Directions.Waypoint({ address: 'Pretoria', location: new Microsoft.Maps.Location(-25.746000, 28.187100) });

    var waypoint2 = new Microsoft.Maps.Directions.Waypoint({ address: 'Johannesburg', location: new Microsoft.Maps.Location(-26.201500, 28.045500) });

    directionsManager.addWaypoint(waypoint1);
    directionsManager.addWaypoint(waypoint2);
	directionsManager.addWaypoint(waypoint1);

    // Set the element in which the itinerary will be rendered
    directionsManager.setRenderOptions({ itineraryContainer: document.getElementById('printoutPanel') });

    directionsManager.calculateDirections();
});

This is the route generated an hour ago.

This is the route generated a few minutes ago.

You can see that 2 different routes are generated. The second image is the best route and the reason it was different earlier appears to be traffic conditions. I was under the impression from the documentation that the route would be calculated to minimize the time and would not take traffic into account.

How should I setup the directionsManager to get a route that does not take traffic conditions into account?