well confused

i have the zebra skin box ticked, and when i click on its name it shows up, which is what i want, which is great, and the default is not ticked, the forum is right for me but its still the default for everyone else!
I tried deleating the default skin while having the zebra one ticked and it said
'The default style set within the vBulletin Options can not be deleted. Please change this before attempting to delete the style.'
i dont get why it works me and my friend who have been through admin but not anybody else!