Abortion isn't an either or position. There are quite a few position and this stat only shows a single one: 15-20% believe it should be illegal in all cases. What about the subset who believe it should only be legal for rape victims. Or only plan B style drugs (I'm not sure how the linked data classifies the responses, but some people do consider such drugs on the same level as abortion). Or only by first trimester. Or only by second trimester. Or until birth? Or some other position that I am not able to remember off the top of my head?