I have just recently starting doing deadlifts, but here is what i am guessing. The dead lift is a two part movement. First, you lift the weight from the floor to about the knees, then once the weight passes your knees you bring your hips forward and extend your knees and hips.
I am guessing that the initial phase is more back to get the weight past the knees. And then once you get the weight past the knees and can bring the hips forward, this brings more thigh into it.
Of course, the whole time, the back has to be working and stabilizing and pulling.
But i have never actually "done" a deadlift, so take this explaination with a grain of salt.
-Fatty