Circle of Confusion

Most images produced by cheap cameras have a high level of sharpness in the center which gradually blurs toward the edges of the film. I'm sure you've seen the effect in pictures made with the popular Holga Camera. This old timey Duaflex that I've been using seems to have something else going on. The center of the image is out of focus. Then the blur dissipates for a while leaving a ring of focussiness. Then it gets blurry again on the edges. I've illustrated the area of focus in the animated gif above. I guess it is possible that the subject in the center is just too close to the lens, inside the camera's limited depth of field. But I see similar focus rings in other pictures taken on this same roll of film where the centered object is much further away. I don't know what's going on, but if I think about it when I'm out shooting, I might be able to use it to my advantage. Any optics experts are welcome to jump in here and suggestsanswers to my question.