Unusual? I'm not certain why there's a question. Your client contracted with you for the preparation and service for a specific number of diners. You served more than those for which she contracted. By law, in most locales, she is obligated to pay for the extra plates, unless in your contract or informal discussion, she clearly noted that there could be additional diners, and that these would be gratis.
The real question, however, is whether you want this individual as a client in the future. If you press this issue for the additional payment, you know that you'll lose her as a client and get some unfavorable word-of-mouth. It's a situation that most caters encounter endlessly, unless there are specific contract provisions and they are clearly understood by the client.
Sadly, there is too often the lack of understanding by our clients that the cost of the food, alone, is a small portion of the plate cost. Once they are educated to be fully aware that there is a significant cost of labor per plate, situations such as this will diminish among reputable clients.
Frankly, too many clients underestimate the number of plates with larcenous intent; particularly if service is buffet. They actually assume that we won't know how many are being served.