New encoder motors not in sync

I have a brand new PiTop 4 with Electronics kit and Robotics kit. We are following the instructions to build a Rover on Further (Robotics 101). When driving the robot forward or backward the robot does not go straight. It looks like one of the encoder motors has a slight delay when starting and stopping. Once they start moving it looks like they move at the same speed, but the delay at the beginning and end makes the robot turn when it should be going straight. Are these motors supposed to be in sync when using the forward command or do they somehow need to be calibrated before use?