Treadmills are treacherous.
Why is that? Let's compare it to the track.
The track is fixed length and fixed elevation (should be completely level). Everything is known. You are subject to weather, but that's just outdoor sports.
The treadmill on the other hand has a lot of unknowns/variables.
1. Calibration. The speed displayed doesn't necessarily reflect true speed. One should always do a calibration, or at the very least check for differences between displayed speed and actual speed for every 1kph / 0.5 mph in the relevant area and adjust settings accordingly.
2. Calibration 2. The effect of your own weight on the band may not be accounted for in the calibration process. The effect could be non-existent, or it could slow the band down ever so slightly.
3. Speed over time. The speed of the band doesn't necessarily remain true over time, much depending on the motor. Weaker motors may cause the speed to drop off slightly (and proportionally) over longer runs.
4. Incline. As with speed, the incline displayed doesn't necessarily reflect true incline. My own treadmill was supposed to support 0-15% over what I assumed would be 15 steps. After measuring with the help of a stabilized laser I found it to be 2-10.5% and over 20 steps. 0% incline in the settings (with me standing on the impact area of the band) was actually 2% incline. The impact of a running stride (as opposed to standing still) bends the plate down ever so slightly, but nowhere near 2%.
5. Incline over time. In longer runs you could effectively stomp the incline down, depending on the motor/system.
6. Longevity. Calibrations need to be repeated every once in a while.
If you're doing treadmill sessions and feel that the performance is too good to be true, or that it feels way harder than it should, the mill is probably uncalibrated.