Close one eye and hold something round out in front of the image until it matches the size of one of the shapes and you'll see that they are indeed quite circular.
This is called a phase-shift or Popple illusion.
Here's a variation:

If you want a real explanation, it'll cost ya...
http://www.sciencedirect.com/science/article/pii/S0042698904003694
The eye is easily fooled. The rings below are concentric.

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