Apart from the 'protocol' of using the first frame when the ball has left the foot - which clearly in this instance undermined any claim of millimetre levels of accuracy in the process, the other factor I haven't seen in play in this is the selection of Mina's foot as the rearmost point of a defender; they use the 'centre of gravity' process to determine where the vertical line from Mane's shirt sleeve would cross the ground, (which again introduces marginal errors), but no similar analysis was applied to Keane's backside which on a simple look could as easily push the defensive line back a fraction. Moreover, they probably couldn't do that calculation, because you can't see Keane's feet on the freeze frame, needed to decide where the vertical meets the ground - if that is the case, then the actual offside line was indeterminable, and the one they went with was, by definition, an arbitrary choice.