Ruger found that die injecting such large stocks out of the Zytel material would create voids in the final product. This is the reason Ruger made the boat paddle M77 and 77/22 stocks the way they did. Through the years somebody figured out how to add those voids to the mold, so then Ruger came out with their normal butt stock design.
And yes, the voids do also reduce weight.
Chet15