Yeah, I was wondering about that. Apparently what you really want is a crank arm forged to the specific length desired, and then machined appropriately, rather than machined from some larger forging.
Yeah, here is a piece on this,
Forging, Casting and CNC Machining
I'd always thought that cold forging was the best. There's a discussion on p. 42 of the 1992 b'stone catalog on this which is pretty good:
http://sheldonbrown.com/bridgestone/1992/1992.pdf