It is far easier to tell a lie than you might think. If you were to take a careful look at your conversations you might be surprised at just how many you tell.
Lying can become such a habit that you don't even notice yourself doing it.
You can see the work of the Spirit in the person who once told lies, but now they will feel so terrible if they even embellish the truth a little. In later steps of this part of the sanctification journey, this person will relax more as they don't need to guard their tongue quite so closely. Honesty has become an instinct that will grow even stronger in the future.
How is this different from someone who is not a Christian but is honest to a fault? It's a question of motivation. God's glory versus man's pride in himself. Or they are not as honest as they think. Some people never have to transform in this way though, and that is just how God has made them.