why do photons fall twice as fast as other objects?

Let's state this more correctly: the bending of light according to GR is twice the bending of light according to newtonian mechanics. The last exist (I didn't know it until some months ago) and can be computed making the limit for mass-->0 of the bending of a particle's trajectory.

So, it's twice with respect to other

*non-relativistic *objects, not respect to every other object.

However, I don't know why this factor two. And I really would like to know it too!