Yes, no one knows how it really works ^^ though there are some theories about it : https://us.forums.blizzard.com/en/d3/t/clarificationthoughts-about-the-atkdef-system-in-d4/18778
In my opinion it could very well be a simple relationship between Attack and Defence, where your Attack is divided by the target’s Defence. That way, if your Attack is higher, your ability get a bonus damage, if it is lower you loose some. For example, 3 600 Attack vs 3 000 Defence = +20% damage.
Kind of like D2 Attack/Armor rating but with higher/lower damage instead of chance to hit.
I hope Blizzard clarifies the matter in the next quaterly update because I see people still mistake Attack/Defence for D3 DPS/Thoughness.