Me and my brother have problem with armory level's

Now as you can see…i cannot take you a screenshot but i will explain you. So here was how is goin
My armory level 6 unlock’s when i reach arena 10 right?
So the problem is that my brother need’s arena 5 TO UNLOCK ARMORY LEVEL 6 (soz for caps)
Please explain me why does that happen?

Sorry, can’t help… I’m pretty sure that older and more experienced players also can’t help you because they don’t have it either…

If you do… tell me how to get it

If i only talk member’s of tactisoft then that would be perfect.
If you are here pls message me.