1) LiteOn was originally a +R burner. -R was tacked on later on. Much like the BenQ, LiteOn has a bias towards -R.
2) The -R media market is flooded with crap (Princos, etc.). If you compare good +R with good -R, the difference won't be very big. But if you compare your typical +R with your typical -R, the difference will be bigger, because the typical +R is of better quality.
3) +R is a technically superior & more robust format, and this makes it easier for a burner to get good burns on +R.