Good edit. That's typical. I just can't figure out why they would bond two pieces of aluminum together, to get the same net shape and near identical mass as the 1-piece cast aluminum 105 spider and crankarm. UNLESS... it's 2-piece to make the arm hollow? But it's only 9 grams lighter. Maybe the ultegra came first to market with that style, then later with the 105, they decided to be a bit less clever, make it 1-piece?