Why are appetizers served in prime number amounts?

I've found that at many restaurants, appetizers can often be ordered only in prime numbers, and it is so frustrating. I know this seems arbitrary, but when you think about it, serving food that is meant to be shared (i.e., divided) in amounts that by definition can not be divided more than once seems ridiculous and counter-intuitive. What is the reason for this? It happens way too often to be a coincidence. Maybe it makes parties order two servings? Please answer if you can! via /r/food http://ift.tt/2sSbl8C