"Why" is a good question. I believe that it's important to look at all eight of the functions and how we use them. There is a misconception that we only use four functions. In fact, you have the right idea, that if you are really strong in Fi, you're automatically going to be strong in Fe. However, the ordering of the functions is not wrong. The ordering has to do with personal preference and conscious awareness. Automatically Fi goes well with Te, so that will be apart of your function stack. Each function pairing groups together and forms that preference. But strength is an entirely different concept that isn't discussed enough. You can also grow in your shadow functions, but you're still one type. There is a lot of variability within a type due to the fact that we use all eight functions, and how the conscious functions work with the shadow functions. In order to grow within your most preferred functions, you will automatically be growing in your shadow functions, becoming a more balanced human being. This is why the stronger your type is, the harder you may be to type. Or if you are really unhealthy and neglecting what you're good at it, but utilizing your shadow functions, then you will also be harder to type.