Bartholemew is correct regarding fatigue. It is insideous and quite surprising what your mind will allow when heavily fatigued. That the crew did not recognise their error until too late is sad and maybe surprising to some however I have made some mistakes suffering under the effects of fatigue that I could not believe at the time. In one instance I put fuel in a tank that was snagged for a leak and MELed to home base. This was not picked up by anyone else on the crew and I first realised it at top of climb.
Dave Gittins
With the wide weight range that is possible with large transport aircraft V speeds can vary by 30 knots. If the Vr speed was within the range that could be experienced in a light weight B747 it is possible, given the fatigue, that no alarm bells rang in the minds of the crew.