Because there is often disagreement about the meaning of a requirements Statement, focus on the costs of the work and how your new understanding of the requirements impacts the plan. If the customer’s interpretation is correct, as it often is, and if your plan was based on different assumptions, make sure that everyone understands the change's impact on the plan. Even if your project will have to "eat" the added cost, get the issue on the table as soon as you understand it and let management handle the cost and schedule problems with the customer. Again, the PSP planning methods will help you with this strategy.