Question Regarding Forum Icon

I see what most of the forum icons mean at the bottom of the page, but one can't find. The icon with a check mark on it. What does that mean?

BTW, sorry if this is in the wrong section.

iirc, it means you posted in that thread

That's what I was thinking too, but according to the icon legend:

You have posted in this thread

And I see this icon correctly, but sometimes I see the one with a check-mark. This one:

They're all messed up... I hardly ever get the right icons for threads I've posted in...

Ah I see. I'm still curious why that check-marked icon exist though...