Some times the camera is not well placed and it is not 100% horitzontal. Then there are mistakes when placing the coordinate system and in some calculus. Drawing a line horizontal or vertical using references in the movie and then place the coordinate system aligned with this external reference could solve this problem.

Another thing is to calculate inclinations or an angle between to segments that are not intercontected by an axis of movement. Then an angulus of "four" points would be the point. Digitization is then from distal to proximal in both arms.