Maybe not science but there was a system of reasoning used to arrive at 1320# meaning it wasn't entirely arbitrary. Briefly, they wanted some legacy airplanes to slip in under the wire without allowing the new category to be flooded with existing airplanes. It was a compromise intended to promote innovation with new products while existing legacy planes provided an initial boost in activity to get the ball rolling.
Whether or not it was a good call will be debated forever. Regardless of what weight was chosen for LS aircraft, somebody would have found fault with it.