i thought it was because the functions we use determine our type
Well yes, that is true. But if I were to use this test as the basis of determining my type, my type would be changing throughout my life. (actually, I don't necessarily have a problem with that theory either ;-).
I have been heavily Fi-focused the past year or so, so that's why I came out ISFP that one time, and most of the rest of the time INFP.
Had I taken the test when I was 24/25, I am pretty sure Ti would have been the highest, because I was really focused on that at the time, so would have responded to the Ti-slanted questions in a stronger way.
So for me, I don't think my 'testing' with Fi at the highest means a whole lot, and I don't think it means I'm therefore Fi-dominant. It just means I'm dabbling in that function more of late.
"...On and on and on and on he strode, far out over the sands, singing wildly to the sea, crying to greet the advent of the life that had cried to him." - James Joyce