Well, to the OP, not sure if this is exactly what you were asking, but:

FPS is typically measured during a relatively static time in the game when not a lot is happening on the screen (barring the lagfest known as Dalaran). If you are sitting at 100 FPS at that point, then when you get to a place where the load on your graphics card is much higher, your FPS will drop. You have a lot farther to drop at 100 FPS baseline than at 35 FPS baseline before things start looking very choppy, so to me it is about the ability to handle temporary increased loads at certain times (hello Wintergrasp).