The common wisdom I've heard is that since 18" is the legal minimum barrel length, they add 1/2" just to be safe in case the police measure it the wrong way.
But, rifle barrels come in at exactly 16" and they don't add an extra 1/2" of buffer to those.
People don't drive 5 MPH below the speed limit to be "safe"
Why are shotgun barrels the one thing I've seen that has a little extra margin of error added on? And why is it 1/2"? Why not 1/8"? or 1/4"? How did we decide that 1/2" was the perfect margin of error to add to this single firearms product?
Edit: It looks like the Shockwave comes in at 26 3/8"... so they've added a little buffer on the end of that, too, whereas 26" was the required minimum size.