my husband has lied to me many times , sometimes i understand that he is doing it because he knows he did something wrong and doesn't want to admit it but sometimes i see him in conversation with other people and he lies or exaggerates for no apparent reason. for example someone will ask him "how many people were at the party u went to last night ?" there were maybe 30 people there and he replies 100. once or twice i corrected him and he got annoyed with me. he seems to lie for no apparent reason. sometimes i catch him in a lie and show him black and white proof that he is lying to me and he still cant admit it ,whats that about?
I think a lot of pwBPD like to exaggerate or lie to fit in? They often have low self esteem and want to connect with people, maybe the story is more interesting when it's a bit more exaggerated? Impulsivity and intolerance of boredom are supposed to be characteristics of BPD too, so maybe it's a spur of the moment thing to make life seem more exciting rather than empty and nothing which is what many of them feel deep inside. For BPDers, feelings are facts, so it is very difficult for them to override that even when you show them b&w proof, they need to deny it to keep their worldview intact.
Also maybe it's just a brain wiring thing. BPD have strong emotions, so that definitely can affect memory. If they had a great time at a party, he might actually remember in his head that it was 100 people instead of only 30. Memory is elastic and changing, this isn't just a BPD trait.