Ok, I made a random tower, and it looked horrible. I simply ragequit and deleted the world. Then, I generated a new one. IT WAS STILL THERE! The world had been re-generated, exactly as I had deleted it. I repeated it about 3 times, and it kept coming back. It's fine now, but has this happened to anyone else? Why does it happen!?
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Ok, so even though I'm seriously pro at all things Minecraft, the fact that I suck at making cool looking buildings means everyone thinks I suck. So I kill them.
If you delete a world, it's not actually fully deleted. The folder is still there, but is unaccessable to you from Minecraft. By making a new world and typing in the same name as the folder you deleted, you are using said folder instead of making a new one. And because you are using that folder, your world gets regenerated regardless of what seed you put in. (This is actually good for worlds that accidentally get corrupted, as it helps bring them back.)
The best course of action is to delete the world, go to your minecraft saves folder and delete the folder from the saves folder. After that, go back into Minecraft and make a new world, and it should be different now (hopefully).