Lucky Chest Price Raise

As I started playing the game, I noticed that my lucky chest price has went from 3 gems, to 4 gems, and now currently on 5 gems per chest opening. My brother is lvl 17 and his chest is still 3 gems a piece, im lvl 12. Does anyone know why this is happening? I've tried looking for answers but literally every forum post, or youtube video I came across never provided them, everyone I see costs 3 gems. Im on android.

Heres a picture just incase its useful lol
Sorry for my noobness.

That's very strange because it has never rose as far as I've seen. Do you spin the chest a lot?