I've been working in the industrial manufacturing process for the last 8 years with 4 different companies. These ranged from a heavy truck hardware manufacturer to the company that make M16's and other weapons for the US Government, FNMI.
Everything that's made has a tolerance. The tolerance size depends on what the item is used for. The truck parts would be as big as +/- [plus or minus] .030 and as small as +/- .003.
On the other hand, the weapon parts were much closer. Some items would run +/- .00005. Any parts not within this range were discarded or reworked.
Tubing and pipe are some of those items that can also have a percentage tolerance. Like wall thickness is a percentage of the diameter. Or the inside diameter is a percentage of the outside diameter.
The tubing the flatline barrels are made from probably comes in long sections and is cut to length, checked for size, bent, checked for size and then bead blasted on the inside and checked again for size. It would most certainly have to start out larger than .6800 because it would need to be that size in its smallest bend to allow a .68 ball to pass through. Metal does strange things when you bend it. The measure we get on either end is not what is in the rest of the barrel. I'm sure they have a liberal tolerance due to the variation in paint sizes. They know and we should know this is not a rifle barrel.
This is why one person gets a super barrel and one person gets one that sucks. Remember, when a green car breaks down on you that doesn't mean all green cars are bad, just that one. This same thing goes in life.
Keep in mind the paint also has a tolerance. After it leaves the controlled environment of the factory the paint changes. The amount varies by different amounts. Think of the difference in shell thickness, fill volume, humidity where you are and a hundred other things that will influence how that little ball comes out of the barrel. I'm amazed that any of this stuff works at all. When you get a combination that works with your setup, stick with it. My rig may not like your components. That doesn't mean you are dumb or you components suck, its all tolerance!