All minimum manoeuvring speeds on the B767 are based on a reference speed, this speed being VREF30. For example, minimum clean speed is VREF30+80, minimum speed at Flaps 1 is VREF30+60, at Flaps 5 VREF30+40, etc. To protect the manoeuvre margin at high weights in the clean configuration, the VREF30 reference speed has had to be increased artificially to allow the VREF30+80 flown to be sufficient/correct. As the VREF25 is only used as an actual approach & landing speed as opposed to a reference speed for other that flap 25 configurations, it is the correct speed for the flap 25 configuration, not an artificial one.
At high weights this equates to a lower speed than the equivalent flap 30 (VREF30) speed. An additional benefit of this anomaly is that in the case of a high AUW overweight landing it is more beneficial to use flaps 25 for the landing as this setting will produce a lower approach speed and will also ensure that you do not get the flap load relief system activating at an inappropriate stage on the approach, resulting in flap blow back (flap 25 limit speed being higher than flap 30).