quote;example: 3/4 PEP has the same I.D. as 3/4 Iron pipe
You are the one who is confused. I.D. has NOTHING to do with being cts or ips. The O.D. is the determining factor. In fact, keeping the copper tubing I.D. constant is the reason the O.D.s are different, because of the variation in pipe wall thickness. Plastic pipe, for a given size, however, will have a smaller I.D. because the pipe has a thicker wall, but the O.D. will remain constant. If the fitting is 1/16" too large, the components, especially the stainless steel ring, will not tighten properly.