There are a couple things that are causing this apparent issue. When I first started using my RCBS / L.E. Wilson tool I bought over 30 yrs. ago, I had brass mouths I could shave with, they was sharp, and I in fact cut myself more than once on them. I just couldn't seem to get a smooth surface on the outside of the mouth, yet the inside of the mouth was always smooth, no lip at all, didn't make any sense.
I tried different tools, yet the problem still presented. But after some time and lots of practice, I figured out what I was doing wrong, and what else was going on, as follows:
If you use a super light touch, and at super slow speed with the drill, it won't leave that little lip your getting. What's happening, the brass is getting piled up above shave line. In a way, when you ream and chamfer brass, your almost literally just knocking the lip off left by the tool, plus a small finishing surface accomplished by the sharp cutting edge if used correctly. And if the lip your referring to is on the inside, your most certainly using too much pressure.
Something else I discovered with many common factory once fired head stamps, and that can fool you into thinking there is still a lip there, is the crimp line left by the factory process. IMO, the worst of them is Winchester, but I've seen this on numerous factory once fired brass. After trimming, the crimp groove is often real close to the rim of the case mouth. So this makes it appear by feel, that there is still a lip, when in fact it's a crimp groove right near rim of the case mouth that your feeling. And it can often be felt on the inside of the mouth also. Short of turning the necks, there isn't much you can do to completely remove it, and even then, some are likely deeper than turning will totally remove. Usually by the 3rd or 4th reloading the brass will stretch enough that it gets trimmed off, most of it anyway, more if my loads are warm enough.
Have you tried seating your bullets with less material removed? And if so, are the bullets seating ok, no shaving?
GS