Here's the real story
Millions of clouds every day
Ok so there are, what, hundreds of billions of possible shapes a cloud could take?
OK fine - the chances are still very good that every so often, one will be a perfect circle or osme other "freaky" shape
The only reason this is so "amazing" is b/c our brains are literally engineered to recognize certain shapes, this just happens to be one of them, so it pops out in your perception.
This has been your psychology lesson for the day. Leave a tip on your way out the door.