Following Distance Versus Stopping Distance

I have been taking the High Road Practice Tests. There is something I try to understand.

I have learned that the stopping distance is 6 seconds and if you drive above 40 mph you add 1 second. Perception: 3/4 Reaction: 3/4 Braking: 4 1/2 The Total Stopping Distance would be 6 seconds or 290 feet. (with air brakes 32 feet more).

I have learned that the following distance should be 4 seconds and if you drive 40 mph or more you add 1 second.

Question: How come the following distance is shorter than the total stopping distance? Shouldn't the following distance be the same as the stopping distance, too?

Drake, following distance takes into account the fact that the vehicle you are following is in motion also. So, even if they hit their brakes to stop they will still have some forward motion going before they get stopped which will in turn allow you, if you are paying proper attention, to get yourself stopped also. I hope that makes sense to you, if not maybe someone else can expound the idea with a little more clarity.

That makes sense.