Note that the short forks are much lighter than the longer ones, the small difference in length can't be the only factor for this so it must also be the wall thickness of the tubing.
My guess is that BSA tried to make the short fork as light as possible by using better quality tubing (Reynolds??)
Nothing special about the material. Just a good quality ductile steel. Try your nearest hydraulic supplier for seamless tubing. Use the thickest wall they have.