I thought so, too. But, I have a brand new treadmill and when I crank it up to 6 mph, the pace is always 9:50 or 9:52 (can't remember). Generally, I look at 6 mph as a 10:00 minute mile.
Whatever; all I know is that I feel way different between 5.8 and 6.0, although outside, the difference is less apparent. I also live in a hilly area, so I do slow down going up the 5-% grades.