Well, it should be obvious that not every feature/user story you develop needs a 40h/week specialist for every part of your product. Sometime you might only need to tweak the UI ever so slightly, or the backend integration could be trivial. Very often you will either have fractional stories in your sprint or underutilize your team. So in a perfect scenario you would share your specialistst amongst teams, but that goes against the typical manager metrics which is "how big is my headcount".
