I am glad you are right for the sake of your counting, but I think using that example, we can see Moe's point about them not being right all the time or estimates being off. If you were eating the bigger size, for example, but MFP just assumes the smaller one, your count would be off. If that happened for one or two other items, every day, I can see people thinking they are eating 1200 calories, but really, they are closer to 1400 or 1500 easily and might never really notice the difference.
Anyone taking it seriously looks at the serving size, though. Plus, you know, labels.
Look, if your calorie count is off, you're not going to lose weight (or maintain it, or whatever you're trying to do). This is not rocket science.
WW actually takes into account the fact that people over/underestimate and that food tracking is not an exact science. I suppose MFP would too. It's more about accountability, I think, than the actual calorie count. Like, it's awesome that you're tracking and knowing what you're consuming and keeping it within a reasonable limit.