Purple gift box prizes

I decided to trade up from my free box and found that the npc won't recognize the box you get when you login and instead you have to ask him for a box which he hands you a Purple Lucky Box which seems to be a completely different box to the Purple Lucky Box you get from first logging in. From the npc box I can trade up, or down if my luck fails. Tho it makes me wonder if the chance to get the better and better boxes is lower as you go.