When the treadmill is going 6.0 mph, the belt is going 10:00 per mile no matter what the incline is. What the equivalents are listing is what effort that is comparable to when running outside. So 6.0 mph at a 1% incline is roughly an equal effort to 9:52/mile when running outdoors. 6.0 mph at a 3% incline is roughly an equal effort to 9:00/mile when running outdoors. The steeper the incline, the harder the effort so the faster the "equivalent" pace.
I'm not sure if this is clear. If it doesn't make sense, let me know.