Dunno if this has been asked, but how relevant is the chest drop %?

I don't know if getting these runes is relevant, i don't exactly feel like it adds up to the drop %. I mean, i tried recording the times that I get boss stage chest and normal chests and here's what i got.

Normal chest drops: Approximately every 5-6 minutes
Boss Stage Chest drop: approximately every 9-10 minutes

Any explanation would help, thanks!