Because the hood stays at the same place and the 24-70mm extends as it gets wider. The perfect hood!
Halfway down
http://www.the-digital-picture.com/Reviews/Canon-EF-24-70mm-f-2.8-L-USM-Lens-Review.aspx
Well you'd be using a 70mm hood when shooting at 24mm.... Plus I doubt the diameter is the same because the 24-105mm fits onto the extending bit not the fixed barrel.
Ahh okay, i now fully undersand
Thanks
Nick