Purely my opinion. In a factory or machine shop...the more operations that can be completed from one 'setup' the better, time is money. If you can place the barrel in the jig, .410 and 20 ga and perform all the necessary machine operations without removing it...you have saved a LOT of time which equals money.
Additionally, if you were to measure the receiver block and water table, I bet you'd discover the receiver is identical for 20ga and .410. So if you slenderize the .410 bbl...what are you going to do with the sharp edges now showing on the receiver block/water table? Re machine the receiver smaller? no way. Follow the money.
