Thats the moment of inertia part, which I will explain here. I must first say that Warband weapons do not run off this, since I did derive the models for this and the glaive comes out with a speed of 68 without accounting for the fact that all the weight is at the edge which would make it slower, also damage solely depends on the weight of the weapon since the speed of the tip being dependant of the length is cancelled out by larger moments of inertia being produced from the extra range.
Moments of inertia is the amount of resistance to circular motion a certain thing has, for a point particle it is mr^2. For something more complex you have to know some calculus (3D calculus is required), you can treat each little mass inside something as a small mass dm at a distance from point of rotation r. Now you could work all those infinitesmal masses and add them together (theoretically), but technically that's an integral. So you take Integral[r^2dm]. This is not very useful however since dm is variable so instead use a constant thing to integrate with. Now density ,p=m/V where m is mass and V is volume. So we can use dV instead. Now rewriting this in terms of dV,
Moment of inertia, I=integral[p*r^2 dV].
Volume integral formed, that is the difficulty to swing an object of length r about it's edge.
Edit: To answer your question the weight at the tip part is "accounted" for in the swing speed of the weapon. Also I said that that equation is for the object of length r about it's edge, but this equation works even if the object is not rotated about it's edge, it's just easier to deal with it that way.