This person sets herself up as the arbiter of what is really meant by the project specifications; contract terms, building leases etc. They usually appears almost as soon as the contract has been signed. Typically at the first planning meeting she will announce that you are responsible for providing some service that you believed the customer was clearly expected to do, at least according to the contract.
I was once on a large contract when we were informed that as consultants we could only claim eight hours a day, but that we would have to work ten hours: effectively giving them two hours of uncharged for effort. The so-called “Professional Day”. This was not just a hit to the company’s revenues it also hit the utilization rate of all the consultants. A key metric for all consultants since it not only drives bonuses it also drives job retention.
In this case the phenomena known as the miraculous shrinking testicles appeared and our management caved and gave them the interpretation they wanted, the deal was too big. In the long term the deal was greatly reduced and the goose didn’t lay the golden egg, it just laid a brick. But we still had our "Professional Day"!
In this instance no amount of political skill or acumen can help you manage this. When senior managers cave into this type of sharp practice then all you can do is salute and either go down with the project or move elsewhere.
In general battling with an interpreter can only be done with the full support of your management and contract people. The basic rule is to make them work for every concession. Don’t agree to any changes without a full discussion and never agree to them without consulting your managers.
More project managers have got into trouble by accepting client interpretations of specifications or contracts than any other single error. We have all been faced with this issue. The survivors have been those who stuck to their guns until their management reached an accommodation with the client. Remember we are project managers not operations executives.