because what happens is when you have an area that curves over, its made by the walls not the top cap. your side/perimeter walls keep stepping over more and more to make the shape of the model. once you get to where to make that shape the steps become so far apart that it leaves a gap then it caps with the zig zag. if you print that with say a 20% infill then that green area should still be solid across when printed. which is what is shown in your preview. it basically caps that area so you arent left with holes showing your 20%. man i think im confusing myself lol, does that make sense

this was fixed in v2.0. in the older versions what you saw in green there would have actually been 20% not the 100% as seen in your preview. i have old prints i did on 1.1.5 with 15% infill where the side walls stepped over real far like that and i could stick my finger into the model. i think they did a good job at fixing the capping and eliminating holes even though it looks odd or give the impression that its not doing it in the preview.