The CAP score is calculated by taking the competitive strength of your team, as well as the competitive strength of your opponent, into account when looking at game results. A stronger team is expected to win a game against a weaker team. Teams that do not meet performance expectations will lower their CAP and teams that exceed expectations will increase their CAP.
For example, if Team A has a 7.00 CAP and plays Team B with a CAP of 8.00, you would expect that Team B would win, on average, by one goal. If Team A (CAP 7.00) wins 3-1 against Team B (CAP 8.00), they would exceed expectations by winning by 2 goals, when on average they would be expected to lose by one goal. By exceeding the expectations for Team A, they would increase their CAP score, and Team B would decrease their CAP score because they did not perform as well as expected.
For an example of what the CAP calculations look like in action, see this sample.