First off, I'm not an AR guy and don't own one and have never seen the buffer tube on one.
This is from the AR something or other...
The biggest differences between the two tube types are: 1) The buffer tube diameter itself 2) The diameter of the threads in relation to the diameter of the tube.
Mil-Spec: On the mil-spec buffer tube… the threads at the front are actually a larger diameter than the rest of the tube itself. In manufacturing, a large tube is used and threads lathed as normal. Next, the rest of the tube must be milled down to the military specified diameter. I’m guessing this step requires more work / precision… and costs more.
Commercial-Spec: In manufacturing, you begin with a tube that matches the final tube diameter. The threads are cut into the front end with a lathe just like any other threaded pipe. I imagine this to be just like the copper water pipe used in my house.
Tubing is measured by it's ID which is constant (to a point) and can have a tolerance OD of "x" depending on size and the wall thickness can have a tolerance of "y"!. These two variables can effect the strength of the tubing involved. From what I can tell, the threaded portion is common to both diameters which explains why there is only one castle nut available.