Yarpy,
You better get used to it. Duties are often scheduled to max FDP. There is nothing unusual in it. As you say any delay will put the flight into discretion and of course that is why discretion exists. Since it is possible to discress up to 3 hours for a single sector flight or prior to the last sector of a multi sector flight, it would be as well to prepare yourself for anything up to a 17 hour FDP ! In the example you give the FDP might extend to 16hrs & 15 minutes. The discretion is actually the "Commanders discretion". This will take into account the circumstances of individual crewmembers so if there is a compelling reason why you couldn't extend a duty it should be brought to the commanders attention as soon as possible.
Unfortunetaly it doesn't stop there. After completing this FDP the company can then shoehorn you into a charter airline seat, an economy class airline seat or into a coach for as long as it takes to get you back to base or wherever they want you next. In such circumstances it is not unheard of to have been on duty for 28 hours or more ! Yet this is perfectly legal. You might wonder what happens at the end of such a duty if you are unwilling or unfit to drive home. Well the company will provide a hotel or taxi for you at their cost, however they will often not consider this any part of your duty for allowances purposes or for subsequent rest calculations (as it constitutes rest of course). Non of this is conducive to prolonged good health but it is legal and it is the framework that many airlines operate to. It is a scandal !