(b) That's right. It is tempting to assume that the parties to a contract will always behave in accordance with its true terms. If this were so, their post-contractual behaviour would be a good indication of what they had agreed. But, despite some contrary opinions, the better view is that the courts will not take account of such evidence when construing a contract.
This is because, firstly, the parties' behaviour may be based on a mistaken view of what their agreement objectively means. Behaviour based on a mistaken view should not generally prevent either party from asserting their actual rights. Secondly, a farsighted party might change their behaviour to influence what meaning a court might later be persuaded to give to particular terms. This would be undesirable.