Yaw/pitch/roll is presumably found by a fusion algorithm contained within the DMP, so it uses both the gyroscope and accelerometer to determine the yaw/pitch/roll. Remember, the gyroscope does not actually determine angular position, but it measures angular velocity. To determine angular position from angular velocity, one must integrate the values over time from the gyroscope to find position. This introduces error, because of noise and imperfection in the sensor. The accelerometer should always read 1G of force in the Z direction when the Z-axis is normal to the ground plane. With this information you can also use an accelerometer to determine angular position, but accelerometers are very noisy so their data alone is not great either. But the fused algorithm presumably is much better.