They knew it was a goal all the time. The onfield team deliberately disallowed the goal so that the VAR could come along and overturn the decision. This devious ploy serves three purposes.
1. It contributes a positive mark to this league table, to make it look like City are benefiting from VAR decisions.
2. It makes Attwell look good for spotting an error and recommending a review. This helps to pad his overall poor refereeing stats with some positivity, and at the same time reduces his unpopularity amongst City fans.
3. It makes Cavanagh look good for accepting his mistake and changing his decision.
It is a skillfully engineered scenario in which there are many winners, but no losers.