I would think that the primary shape is the one you most identify with, while you may have some aspects of your secondary shape, even though it doesn't totally fit your body.
For instance, I primarily identify as an hourglass -- relatively small waist with defined hips, bust and shoulders, but I am a secondary pear because I tend to carry my weight in my hips and thighs, and I'm sensitive to figure flattery on the lower half of my body. My shape doesn't totally match the pear profile because of my shoulders and bust -- therefore pear would be a secondary type for me, not primary.
I think most of us don't fit neatly into one category.