NGUI does not provide the functionality that it sounds like you're seeking. This is something you can easily do yourself. Just maintain two values... "realValue" and "displayValue".
Then you'd do something like displayValue = Lerp(displayValue, realValue, t)
And then its up to you do something like t += Time.deltaTime * scaleFactor; each frame, reset t to zero on value change, etc etc
OR ...
You have "Steps" set to something like "4" instead of being "0"