I knew this looked familiar.
Batman taught Nightwing, and you never teach someone all you know, you always hold a little something back. Batman's spared with Nightwing he has played it out 100 different ways with a 100 different outcomes, and it always comes out with Bats kicking Nightwing's ass.
"BatMan is superior to Nightwing in every respect except acrobatics. All other areas, such as physical conditioning, combat skill, and intelligence, BatMan takes it. Bruce for the win. "
Nightwing is also slightly faster than Bats, a more natural athlete, and his movements are more fluid. NW beats Bats in agility and speed, Bats beats NW in strength, skill and intelligence.