1 of 56 Wonders

Why are there multiple classifications on the 7 Wonders of the World?
7 Wonders of the World
7 Wonders of the Modern World. 
7 Wonders of the Medieval World. 
7 Natural Wonders of the World.
7 New Wonders of the World.
7 Wonders of the Industrial World.
7 Wonders of the Underwater World.
7 Wonders of the Ancient World.

If I'm not mistaken, that makes 56 Wonders, not 7.  I'm not writing this as a complaint by any means, since, through my travels, I've found multiple locations that I wish could be added to the list.  My goal for a while has been to see THE 7 Wonders of the World.  So far I haven't even been able to start that list yet.  However, branching into the categories, I recently saw my second of the 7 Wonders of the Medieval World (the first being the Colosseum). After all, if I was going to be stuck in England, there's no way I was going to pass up Stonehenge. And as a member of the primary list of the 7 Wonders or not, it is an incredible and marvelous structure.

Stonehenge is surreal and much larger than I had ever imagined it to be.  It was believed to be erected between 3000-2100 BC.  Each stone measuring around 22ft high with another 8 or so feet below ground and weighing about 25 tons. And there are an unlimited number of theories based on how people, in an era thousands of years before Christ, managed to not only erect the mighty structure, but bring the stone from distances as far as Wales and Scotland.

I was in awe.