Fuel Burn
It is my understanding that whenever refuelling happened the receiver was topped right up. So for a considerable time the Vulcan was actually above Max AUW, where it was noticeably thirstier. The other factor was that crews were normally operating at between FL410 and FL450. Since AAR was being done in the high twenties/low thirties the engines burnt more and the groundspeed was less. The relevant pages of the ODM were not looked at in my 7 year stretch on Vulcans, as we just did not operate like that.
I am surprised that enough real information was not gleaned from the transit down to Ascension to increase the amount of fuel put in the air on the night of Black Buck 1, but lessons would seem to have been applied on the later missions.