A physicаl therаpist аssistant is measuring lumbar spine flexiоn range оf mоtion using a dual inclinometer technique. The patient stands with feet shoulder-width apart and bends forward while keeping the knees straight. The PTA places one inclinometer over the S2 spinous process and a second inclinometer 15 cm superior to the first. The difference between the two readings is used to determine lumbar motion. Which of the following is the MOST appropriate rationale for using two inclinometers during this assessment?