By the geometry, there shouldn't BE a gap.
I suspect what happens is the center of the 120 degree rivet gets pulled into the 100 degree dimple/countersink after the perimeter makes first contact, resulting in a slightly misshapen factory head and a sunken center around the mandrel hole in the pulled rivet. The outer edge of the factory head may pucker or become wavy in this process. I certainly have lots of examples I could examine under magnification here, as I own no 120 degree dies or CSK and have pulled lots of CS4-4's. Will have to scope that detail out next time in the shop...
Hop-Along Aerodrome (12VA)
RV-6A - N30YD - flying since '98
RV-10 - N130YD reserved - under construction
donating monthly to the VAF - thanks, Doug