[BasilG]

Heard of constructive/destructive interference? The defocus characteristic with a possible bright centre, and bright outline means that along lines of contrast, you can get brighter and darker areas next to it. In the Siemens star, the lines are not parallel so the spacing distance is changing. At times, the spacing will be just right that it boosts contrast, which gives you the what looks like in focus areas. Other times, they cancel each other out and you get grey, like the region between the two contrast rings. I'd consider it a side effect of the defocus characteristic.

 

Actually, what you see here is the contrast transfer function. The grey ring between 1/3 and 1/2 way out along the radius of the siemens star is where the contrast transfer function crosses zero (no contrast transfer). After crossing zero, it causes contrast reversal (or phase inversion). You can clearly see that - if you follow a black line, and extrapolate it through the grey ring, it continues as white at the other side of the grey ring. This effect is caused be the defocus. If you go closer to focus, the grey ring/zero crossing will move and finally disappear when focus is achieved.