They obviously didn't take into account Horseshoe Bend which now sees 1 million visitors a day!
I'm sure you've got your numbers wrong, probably more like a quick Google search would provide 1.5-5 million a year. Still plenty though…
I'm surprised Australia came in at No 10. For most people it is a 20+ hour flight away and very expensive (unless you want to travel like a backpacker).
Cheers,
Cheers,