Why do share prices move on news everyone expected?
Because prices reflect expectations already, so what moves them is the gap between what happened and what was anticipated — not the news itself. This single idea explains most of the behaviour that looks irrational from outside.
The mechanism. If a company is widely expected to report strong profits, buyers have already bid the price up in anticipation. When the strong profits arrive, there is nothing new to learn, and the price may not move at all. If profits are strong but less strong than expected, the price can fall on excellent results — which is the thing that most baffles people watching from the outside.
The vocabulary this produces:
"Priced in" — already reflected in the current price.
"Beat" or "miss" — relative to analysts' consensus forecasts, not to last year.
"Buy the rumour, sell the news" — positions built in anticipation are closed once the event occurs.
Guidance, which frequently matters more than results, because it changes expectations about the future where results describe the past.
Why forecasts themselves get managed. Companies have an incentive to guide expectations slightly low so they can be beaten, a well-documented pattern — which is why a small beat is treated with less enthusiasm than the number alone suggests.
What actually moves prices: genuinely new information; changes in expected future cash flows; changes in interest rates, which alter what future earnings are worth today; changes in perceived risk; and forced buying or selling for reasons unrelated to the company, such as index rebalancing or fund redemptions.
The efficient market idea, honestly stated. The claim is not that prices are always right, but that publicly known information is generally already reflected, so acting on information everyone has is unlikely to produce an advantage. Markets are demonstrably capable of getting things badly wrong; they are simply hard to beat systematically after costs.
The practical implication: reacting to news you read in the paper means trading after those with faster access and lower costs have already acted.
General information, not investment advice.