There is the possibility that the dominant function can be so encompassing of your view and interactions with the world, that you take it for granted and do not recognise it's dominance. It's quite common for people to be surprised to find out that not everyone sees the world through the same dominant lens that they do.
I think the theory is that functional orders, are underlying methods of operation, we can pick and choose functions to use as tool's but the overall method we use is supposed to be ingrained.
Your dominant function (to me) is natural, instinctive, all encompassing and constant. People are often better at describing their second function than their dominant as they have no reference point to describe their dominant from, it IS their reference point.