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Many times in astronomy, when we look at a hollow spherical cloud (a bubble) we get the impression of a ring. Often this is an optical illusion, as illustrated in the above figure showing a bubble in cross section. When we look at the center, we are looking at a short path through the bubble, but when we look on the edges we are looking at a much longer path through the bubble (the blue portions of the paths in the above figure). Thus, the cloud appears darker in a ring around the center.
In the case of a planetary nebula, the expanding nebula is usually a bubble (though often not spherical). But because we look through less gas and dust when we look at the center than when we look at the edges, we get the impression of a ring when we observe it.