As I understand it, there are no "race modifications". A bonus setting at startup that is available to the AIs is available to the human player, and I don't remember one for planet quality (maybe I missed it). However, there is an event that any player, AI or human, can experience that boosts the initial quality of planets when you colonize them.
Another possibility is when you invaded you have a number of attack options. Some of these can degrade the quality of the planets in the system, and some can destroy improvements the planets have built. (If it is because of destroyed planet quality improvements, I would have to ask Stardock to check on it, because the PQ improvements do not sound like the kind of improvements that can be destroyed.)
~SDC~