The Model 90413 sensor measures steering angle using a bi-directional MEMS inclinometer.  The inclinometer is oriented such that one axis measures the steering angle directly, while the other axis measures the tilt angle of the wheel.  When the steering wheel is tilted, the sensitivity of the steering angle measurement changes.  This is not a property of the MEMS sensor, but is the result of physical reality and can be computed using geometric principles.  The measurement of the tilt angle allows the Model 90413 to compensate for these changes in sensitivity, so that the sensor measures accurately within its range of tilt and steering angle.