Unfortunately, I don't think it had anything to do with the Combi versus Passenger derivative issue, as Boeing gives the "typical cruise speed" as 565 mph for both models.
Just for grins, if the route were the same (i.e. 5836 miles via great circle) the speed of the passenger would be 526mph and 446 for the combi. This, I believe would rule out differences in the weight/speed or engine suggestions previously given as the difference is just too much.
KLM also uses the combi on the IAH-AMS route and the average speed developed on this route is 536 mph (great circle = 5004 miles and duration is 9:20)
This therefore, leads us back to the routing. Does anybody have suggestions as to the large difference? Seems very incredulous that on one day, it takes two hours longer which could be equated to 1000+ extra miles on the flight path.
Look forward to additional responses