No it's not a glitch, it's just how smf works.
I'm guessing the database checks your permission when you set the signature and not every time the page loads or after a specific time period and this is for efficiency and a bunch of other reasons.
I have always follow your comments and learning a lot to it.
Probably just like this guy had said that the page loads after a specific time of period.
As I remember when I was new I always wanted to updare every posts I made as activity but it only happens im every 30 mins. I guess before I can check my posts updated as activity count. This is what I observe but I do not know why it happens like that even in the time I awarded merit to someone the number will not change immediately after the merit awarded ita may take 5mins. I guess I do not know.