My guess is they've bought the picture from another shoot. If it was their shoot it would be "Sun World Exclusive" and they'd have made a lot more of it.
I expect the full shoot will appear in Nuts soon!
Good point - they've watermarked the picture though, so I suppose it's possible she's done the picture for a specific photographer maybe in an attempt to be 'arty'.
I imagine Nuts or Zoo will pay a small fortune for the rights to print it though