Myfitnesspal/Nutrition Question

I use myfitnesspal.com religiously to track my every bite.  I went into the custom goals section and have changed my goals to my personal/doctor goals.  This means 100g protein & 50g carbs.  Why does it automatically change your carbs/protein/fat when you change one of them? 

This is hard to explain.  But I mean, when I change the "percent" of carbs, it changes the percent of the other two.  They always have to add up to 100%.  It seems to me that this is telling me that all calories have to fall into one of the three categories?  Is this true?  If so, in meeting the carb limitations and high protein recommendations, it leaves your fat pretty high.  Is that normal?

Thanks for any suggestions/input!

I think it is based on "Normal" people instead of WLS folks.

 every calorie does fall into one of the 3 categories because that's what they are made up of. and it is annoying how they all change on there when you change one, so i stopped setting the goals and just look at my daily totals

I've always had the same problem - I just set mine so I got it as close to 60g protein and 50g carbs every day as possible, as I don't really have a calorie or fat goal at this point (I'm about 6 weeks out).