Our state soils extension specialist has said research shows that when soil P levels are very high, it has shown that soil P is more mobile than what we used to believe.  And I believe that is what you are referring to. 

However, I would still say that K is more mobile than P.

One thought crossed my mind as I read your post......  is it the fact that the P is mobile, or is it the fact that the P is attaching to particles,  and is being moved in the process......  kinda like the process of erosion?

We always say that the way to keep P in place is to no-till.  More Soil P is lost through soil particle erosion than is lost thru movement down thru the soil.....   at least that is the way I understand it.

martin