I've head this comment before, but from my own speech I don't find this to be true at all. It usually means I'm saying something I perceive to be more vulnerable or direct than any party to the conversation has been to that point - not at all that I was being deceptive before that point or after.

> vulnerable or direct than any party to the conversation has been to that point

You seem to be sort of acknowledging the contradiction in this one. If the conversation is guarded (self-protecting vulnerability) or indirect, then it isn't being frank.

But what follows Frankly is genuine whereas in the other examples the target phrase is used to reinforce a deceitful stance.