Confused about 600EX's flash range graph

Hi,

It's my understanding that the graph display on my 600EX is meant to represent what range the flash would work correctly at. That is, when my flash has a black bar between 20ft - 60ft, and it says 20ft-60ft @ 2.8 next to it, it means that being closer than 20ft would be overexposed and being farther than 60ft would be underexposed?

If that is the case, why when I change the flash compensation from 0 to -1 the range actually gets higher, say from 20-60ft to 40-60ft? Shouldn't the range get nearer?

Confused.

When you change FEC to -1 the flash has 1 stop less power required therefore it's range will be extended 1.4X what it's range was at 0 FEC. IOW it has one more stop of power available at -1 FEC.

The scale is suggesting the range where the subject will be properly exposed with ETTL metering.
-1 FEC lowers the power requirement. The near number will increase because it is indicating the lowest possible power output from the unit. The far number will increase because the max power from the unit will reach farther at the lower exposure.