That's the thing about chronic liars. They think they're good at it.
Perhaps it's a practice-makes-perfect kinda mentality. So often so many think they've got it wired.
When the opposite is so often true. They lie so habitually and they think so little of it that inevitably they can't remember who they've lied to, or what they've said. So that if you're paying attention at all you more and more catch them out, until nothing they say is believable on face value, and the relationship comes to seem more trouble than it's worth.
Or you come to hold them in total contempt. Another consequence they seem unable to foresee.