You're loading those various brands of cases the same, you're very likely going to get different results for each, because . . . they are not the same. There'll be difference case wall thicknesses, which effects the volume within the case and variable volume creates variable pressure's which results in variable MV's. Also, the brass of each are just a little different alloy making one a little softer than another which effects how they respond to expansion and contraction. For example, Federal's brass tends to be softer than most and one of the reasons reloaders s don't get as many reloads from them. If you're just plinking and/or just practicing reloading, then this detail doesn't matter.
As has already been well stated, the BIG advantage with reloading is in producing that are consistent and tuned to you're particular gun (e.g. controlling power charge and details of the case configuration). . . . especially for the likes of a 6.5 CM. For many people, the other "BIG" advantage is reducing the cost of each cartridge when shooting large quantities of ammo. That a case is "formed to your chamber" is NOT as "BIG" advantage as several other pieces of the reloading process is much more (like powder charges).
For many, just FL sizing their cases is simply an easier way to get uniform cases that work well. AND, a FL sized case provides less chance of a stuck case in the chamber (a big issues if one is competing or hunting in poor environment). Neck sizing only (and when I say "only", I'm not excluding having to bump the shoulder back each time - which I do) works just fine and prolongs the case life where it can be reloaded more times than if one is FL sizing each time which you're not hard working the case body. What can be another advantage of neck sizing is you have a case volume that's larger than when when if FL sizing. The extra volume allows for little larger powder charge in the event that one is trying to achieve some accuracy-maximum velocity load.
If one has a mass produced factory chamber, neck sizing along with shoulder bumping provides a case that is more of a custom fit, though it takes more monitoring as a factory can cause more stretching of the case where at some point the case will need FL sizing very once in a while (noticeable by a stiff chambering of the cartridge). Many competitive shooters (depending on the type of shooting they do) have custom chambers as well as custom dies and simply do FL sizing, and in rare cases in some custom chambers, sizing may not be necessary at all.
Now, are you totally confused yet?