Why is Adobe Illustrator rendering my concentric circles like this

I'm not a professional user of anything Adobe makes, but I am a programer and I do understand math, which is probably why this hurts my brain. I open a new empty Adobe Illustrator CC document, set the dimensions to 250px x 250px, set the Artboard center markings on, place my cursor and create a circle 25px across, I carefully use the transform tool to move it -12.5px x and y, and then make my next circle, this time 30px across, and transform it -15px x and y, and I notice it looks wrong. I loose 15 minutes mucking around with the idea perhaps I turned something stupid on with perspective then give up and try to reproduce the 'bug'. I make 2 new circles, each time doing a right click, and transform manually so that they are centered, then move it to the back of the layer stack so the circles in front are visible.

I get a weird "saggy bulls-eye" effect like each circle has warped either high or low and I have no idea why. Explanations?