Improved balance and perfect balance only reduce the two-weapon fighting penalties for *actual* light weapons. Even with perfect balance, if you're using a one-handed weapon in your off-hand, you get no penalty reduction from Improved balance or perfect balance.
Those two abilities do the same thing, it would have been better to word them thusly:
Improved Balance (Ex): The two-weapon warrior's attack penalties for fighting with two weapons are reduced by –1 when wielding a light weapon in his off-hand. The two-weapon warrior can wield one-handed weapons in his off-hand as though they were light weapons, but gains no reduction in attack penalties from this ability for doing so.
Perfect Balance (Ex): [as above, plus:] This benefit stacks with improved balance.
If thats the case, then whats the point of Perfect Balance? Improved Balance says twf is reduced by 1, ALTERNATIVELY, he may use a one handed on off hand as if its light. Perfect Balance says stacking reduce of another 1, (no alternatively this time) He treats one handed as a light on his off hand. "He uses normal light weapon penalties" because twf is not an automatic ability to the Two Weapon Warrior.