Possibly just another 'marketing ploy' to gain the confidence of the gun buying public.

Lightening cuts add to the cool factor. The weight savings are a secondary concern. BTW, I think that measuring, and calculating the weight is the way to go.

I also think it was a marketing ploy. Not only did they make the cuts, they polished those cuts so they are like glass. How they did that without rounding the top edges of the cuts is beyond me. I have a top action 16 O frame with the cuts and you can almost cut your finger if you slide it along the edde. Meticulous work.

Many of the ones I have seen are not completely smooth. There is still some ridges in the bottoms of the grooves.
But, it really does look like they plunged while making the cut since the depth decreases evenly by the ends.

They may have made the rough cut with a vertical mill, but look closely and you'll see grinding lines running parallel to the cut, which indicates it was finished off with a grinding operation.