I had noticed on the SrStaff ones. It just shows when we became a senior staff member. Nothing with when I became a mod originally.
Another thing would be that I actually first joined EMC on Nov 8th, 2011, but only joined the forums on Dec 21st (it actually says 22nd on mine?) because you didn't have to join the forums right away back then, and it took me a while to realize they existed.