Yes we all access our variants at different times, depending on what we need when we need it. I guess it's which ones you use most frequently which define your type. But the thing is, this has varied for me at different times in my life, depending on what my primary need is. I think that's what is so confusing. For example, I just started taking a drawing class this week. And when I am working on art, or any type of creative expression, my SP four becomes very strong- I lose track of time, all of my focus goes into my art. I could care less what's going on socially, who else is in the room, etc. I just want to create. But... as far as my life goes, big picture-wise, I would not jeopardize my relationships for the sake of my art. Also, I am very erratic when it comes to my creativity; it comes in fits and bursts. So that leads me to believe I can't be SP dominant.
I think that's how you determine what's dominant- you look for the general themes throughout your life, and what you find you tend to focus on, work on the most. Conversely, in what ways do you seem to be deficient- what do you wish you were stronger at, but you have difficulty sustaining your focus on, long-term? Chances are that's the lowest rung on your variants.
This is my interpretation; I'm really not sure it's accurate.