The instrument has no way of telling how easy or tough it was to make. That could be an indication of tension between what you want to be and what you are asked to be in your job or some other part of your environment. It could be that you're in the mood the day you took the test to express that letter more than you normally are. Many things can alter the expression of that core type. These include environmental pressures growing up, developing functions as you get older, and temporary demands to use a different function than what would be your preference.
People usually feel most comfortable and relaxed when they are using the functions of their type.
However everybody uses all functions at sometime. All intuitives must focus on detail at some point, all feelers must reason something out logically and apply an objective view at some point. Some people have grown up with an environment that discouraged the use of a favored function. They have habitually learned to use a less favored function. Sometimes at a psychological price, since they are living like a left handed person who was forced to write right handed.