Quite possibly maybe from their perspective, the person did not experience their life like you perceived they did.
On the other hand, maybe they are seeking some type of attention from some one else , and by portraying a less than desirable childhood, this is gaining them additional attention and sympathy?
Usually this happens when a person has had a crappy and chaotic upbringing, and they down play the negative in order to make themselves feel better.
There must be something to gain though if they are doing this on purpose and if it did not seem like it was so bad.
However, no one knows what a person is going through lest they walk in that person's shoes for a bit.
A bit ambiguous, I know. However, the only way you will really know is if you ask them about it.