CNC = Computer Numerical Control. A computer and servo motors are hooked up to a mill or lathe. A machinist writes a program to make a part. The machine is very accurate at positioning the tool repeatedly at the same point. You still have to know how to machine a part.
Cast aluminum is a way of constructing parts using a hollow cavity and melted aluminum. Engine blocks are about the best example of a cast part. Casting, when done right, yields very good parts. I think all of the NN3 and NN5 rails are cast (Nick, correct me if I am wrong here).
The cost of CNC is much higher than casting for production parts. Prototypes are almost always cnc'd. The quality is not necessarily lower with casting. Many small gun parts are high precision castings. Car rims are a lot of times cast. Casting is a good way to manufacture quality parts at the lowest costs possible.
Unless you now what to look for, a non machinist will probably not be able to tell the difference between the CNC made swivel and a casted swivel. No worries.
Thanks for that DennisS - we need a post with good explanation of the processing. Many people do not realize why some things have higher cost than others. And then you have to also factor in the cost of buying these machines - they are not cheap!