I don't think I usually mean it as counter-signalling (from the inside, it feels like I'm talking in a way that I find more fun/interesting/funny), but I have little bits of self-depreciation as part of my normal speech. I have to consciously turn it off for job interviews, because in that context it always gets taken literally. This is probably why. (Possibly also some influence from the other person consciously trying to evaluate me during the conversation.)