Line Movement and What It Actually Tells You
A moving number is often read as new information about a game. Sometimes it is. More often it is information about where the money went, and those two things are not the same.
A book moves a number to change the flow of wagers, not to publish a forecast. If one side has drawn the weight of the stakes, the price on that side gets worse until the other side becomes attractive enough to balance it. The size of the move tells you about the imbalance; it tells you nothing on its own about which side was correct.
The confusion is compounded by the fact that both kinds of movement look identical on a screen. A number that moves on a genuine injury and a number that moves because a great many small wagers landed on a popular team produce the same little arrow.
The honest reading of a moved line is therefore narrow: something changed the balance of the book. The interesting question, which the arrow does not answer, is what.