Two motors are driven forward, why are they rotating in different directions?
The "forward" and "backward" expressions are only interpreted in the reference frames of the motors themselves and translate to clockwise and counter-clockwise. In your case you must have mounted the motors in a way which requires them to rotate in different directions in order to propel the vehicle.

Consider a car with four wheels: in order to go forward, the wheels on the left hand side need to rotate counter-clockwise but those on the right side must turn clockwise.

While most chassis types share this property, it is indeed possible to build a chassis that requires both motors to be driven in the same direction for the vehicle to advance. In that case the program must be adjusted to accommodate for that.