The strobe output in TTL just controlls the amount of time the flash is on. The output can only go so low. The control knob on the back of the unit controls the intensity of the strobe.
In TTL, turning that knob down, turns down the range of control.
It happens commonly in macro, where the shortest amount of time is still too strong for the proper exposure, or when there is too much ambient light.
There is a reverse application, where you want to freeze motion... so having the flash use the shortest time, is valuable.
Remember that light reflected off of an object is a square factor (twice the distance = 4 times the necessary light) and both output and time a linear (meaning you have to make bigger corrections than you would expect.
To show this effect...
Get close to something light colored (in macro) and try to use the strobe at maximum.. it will be over exposed. Turn the flash all the way to the lowest and try again...if you get a good exposure, then move out and try until you get too dark... now increase the output until you just get a good exposure...from there, you should be able to turn up the output and still get something good for around 1/4 of the range..
The key thing to remembe is the ttl can only cover a range... you have to adjust to set the range.
