What's the difference between Double cover blue label vs. Married cover green label 1 1

[m19ark]

I was going over my books and I came across a copy of Uncanny X-Men #317 with two covers. I checked Ebay and found similar books that had double covers but some were blue label "double cover" and others were "married covers" green label. What's the difference? I attached pictures of 2 examples. These aren't my books, just ones I found on ebay. 

[Jennifer F.]

Good morning,

Thanks for your message.  A double cover is a mistake in the manufacturing process where two or more covers are bound to the same comic book during that process. Whereas, a married cover means that the cover in question was attached from a different book after manufacturing.  Any time we list a book with a married cover or married pages, it will always receive a qualified label.  

Thanks!

[m19ark]

Got it! ok, now what I don't understand is why anyone would attach another cover over an existing cover. Good example would be the spiderman book I posted with the married cover. Why would anyone do this to this particular book. It's a very common and easy to find book. Why go thru the trouble of marrying it and get a qualified label? What am I missing?