Basil. This also interests me. Sadly I have no answer for you.
I have searched NASA and come up with nothing.
However, in my experience I have only ever seen fuel temps trend towards the TAT (and then remain at TAT). When moving to warmer air again, it trends back up to the new TAT, so it could be below TAT for the time it takes to warm up again.
I suspect this is what they are referring to re 'up to 3 deg below'.
But I stand ready to be corrected by an aerodynamicist.