Seam movement is measured against a line the ball was never going to follow

Deviation figures require a hypothetical straight path, and choosing that path is an assumption dressed up as a reading.

Deviation from what.

Every seam movement figure you have seen compares where the ball went with where it supposedly would have gone otherwise. That second path does not exist. It is constructed, usually by extrapolating the trajectory from before the bounce forward as though nothing but gravity acted afterwards. Reasonable enough. Also an assumption, and one that quietly swallows part of the answer.

The trouble is that a ball off the pitch is not only seaming. It is skidding or gripping depending on the surface, losing pace unevenly, and possibly still swinging in the air, because swing does not politely stop at the bounce. All of those bend the path. The reported number is their sum, wearing the name of one of them.

You can see the consequence in how pitches get described. A surface where the ball holds up and grips will inflate measured deviation for every seamer on both sides, and the match report will say the ball seamed around. Sometimes it did. Sometimes what happened was a friction effect the model cannot separate from a seam effect, because it only ever observed the total.

There is also the question of what the batter experienced, which is not degrees at all. It is displacement at the moment he had to commit, a fraction of a second after pitching and well before the ball arrives. A delivery that deviates a lot over its remaining distance but does most of the deviating late is a different problem from one that started diverging immediately. Both can report the same total. Only one of them is unplayable.

If I were designing the readout, I would give lateral displacement at a fixed time after bounce rather than total angular deviation. That is closer to the perceptual problem the batter is solving, and it is less sensitive to how far he was standing outside his crease.

None of this makes the current numbers useless. Within one match, comparing bowlers using the same system on the same surface, the ranking is probably sound. Across venues and seasons it is far weaker than the decimal places imply.

We have a habit of trusting a number more when we cannot see how it was produced. Seam movement is a derived quantity carrying a hidden counterfactual, and the counterfactual is doing more of the work than the measurement is.