the first couple of weeks after the iPhone 4 was released, obviously Apple doesn't yet have enough refurbished iPhones available, so they gave out new iPhones as replacements. But it has now been 2 months since the release and as people begin to return their phones, Apple are phasing out the new phones and has now started giving out actual refurbished units as replacements.
4 months after I got my iPad, I had an issue with it and I took it to an Apple Store were they replaced it with an iPad from the Brown Box, after I got home I looked at the serial number and realize that even though it was in a brown box, it had a new unit serial number that was manufactured before the one I actually returned.
it just depends what they have available.