I always thought there were many reasons people lied to avoid the truth so they can: avoid getting caught at something, for protection, to make themselves feel more important, to make things more interesting, to see if they can get away with it, they may be lying to themselves, etc. Depends on the person, their past, what they've learned, and the circumstances seems to vary for the reason. I don't know about personality type but there seem to be a lot of factors.

It drives me nuts and if I catch it in time, taking into the circumstances, I will vary how I will call them them out on it now a days as I have no patience for this. If I don't realize it until later, depending on the relationship, it can be discussed later. Otherwise I keep it in mind in dealing with them in the future and pay closer attention then.

It is frustrating to say the least. I am sorry to hear you are dealing with someone that cannot be forthcoming. Word of caution though, not everyone wants to face the truth and can be very hostile when confronted with it. If this is someone important to you, it could be well worth talking to them when they do this that as honest as you are with them you'd expect the same in return moving forward.