when a truthful person tells you their story, it is the only story they have to tell-if you were to challenge their version of events, they will not change the story. They may provide additional information, true, but said information will not conflict with their core story. If you continue to push that person, insinuating they are not telling the truth, they will become angry. Because a truthful person has no place else to go. Their story is the only one they have to tell.