There's a difference between bent and warped. It's not unheard of for rotors to come out of true (warping) due to overheating or rapid heating/cooling cycles, or poor quality, or mishandling. Then there's bent, as when you step on it or bang it down on a boulder or tree stump. Warped rotors you can usually true up using rotor truing sticks or something as simple as an adjustable wrench.
Sometimes, during the manufacturing or packaging process, the rotor will get in contact with impurities that act to reduce friction. On new brakes you'll often smell "burning oil" (Mmmmm... ) as the rotor heats up on longer descents. After a few miles or a handful of hard stops, this will burn off and the brake system will come up to full power.
You can jump ahead by washing your new rotors with dish soap or alcohol. I never do -- just a ride 'er two'll do!