I'd have to see the mirror to see what you are talking about and what you are doing for grinding.
I will note that it is common on subdiameter tools to have the outer edge of the mirror not get ground fully and, while you think that you have the right sag., the spherical curve tendst to be a shorter ROC than that indicated by the sag. as you have dug out the center but haven't made the hole spherical all the way out to the edge.
Also, if you are seeing a finer ground ring that is out about (generally) the 70% zone, that is from grinding a wet too long and this is where the ground up grit has been living while you grind away. Shorter grinding times, a bit more grit (don't forget to recover the grit that falls off the tool with the 80 grit as about 1/2 of it should be doing so for best grinding rates and that is a 50% wastage of something that you need a lot of) and COC stroking will quickly bring a spherical surface to the mirror. If you are at the sag. you desire, you will have to flip the mirror an tool and do TOT to open the ROC of the curve to the edge.