We grow up in our own family, with our family's truths; then we develop our personal truths. Next, we try to blend these with our mate, which obviously (or maybe not obviously) are another family's truths blended with his or her personal truth. Now add in life experience. Of course it's no surprise that there are clashes of "truth."
So what are we to do with this mix of known and unknown or indescribable truths? When we ask a person WHY he did that (which I don't recommend, by the way as it paints him into a corner), he may or may not be able to articulate why; the answer is often it's just how my family did it. And what's wrong with that? You have a problem with my family? No, no . . . Now we're down another rat hole!
Instead of focusing on our truth and trying to convince our beloved, focus on the INTENTION of the communication. I often ask couples, "Do you want to be right, or do you want to be connected and happy?"
So ask a lot questions: What does that mean to you? What does that mean about you? Is that how your family did it? How do you see this? What's your vision about this? How does that sound to you? How do you feel about this?
Dig deep and be curious ? in yourself and in your partner ? to understand your differing truths. I know it takes time. I believe that in the end it takes less time because we don't have to repair misunderstandings.