Due to deck angle
My company recently sent this same question to Boeing for a response.
Their reply was that it is due to the deck angle as it pertains to the two flap positions.
The question came up because company changed the policy concerning overweight landing flap settings (emergency return after takeoff scenario). Flaps 25 is now used at high gross weights, and the slower Vref was noticed. By using Flaps 25 and Vref 25, in addition to the slightly slower Vref, you decrease the possibility of flap blowup, therefore lessening last minute changes in stability due to flaps blowing back down to the set position.