Because using forscan you can see the actual temps and can compare how much the temps still climb once it hits the middle of the gauge on the cluster. There is around a 50 degree climb of actual temps after the gauges hit the center.
So example, around 150 degrees it hits normal and doesn't move again (I'd have to check the exact number.. just an example). Even though it shows mid/normal and doesn't move again, your temp still rises up to abou 200 degrees. It won't drop at all unless you get close to the 150.. and won't rise till you get above 220. So thats why I say it is a dummy gauge, it doesn't actively move with the actual temp fluctuations.