As explained before there isn't room for 3 walls in that section so it only prints 2 and and leaves a gap. This is mostly user error in not understanding the software. To help these people I would maybe suggest a software option to reduce the number of walls around a top section. For the people who want a solution now, you have a couple options to how you can fix it.
1- Print with fewer walls and a higher infill. Your parts will usually come out nicer. I usually print around 50%infill with 3 tops, 2-3 bottoms, and 1 wall. Prints come out great and are plenty strong.
2- Use multiple profiles for different sections of the print. If you still want 3 walls you could print with multiple profiles to achieve nice filled top sections. Use the Cross section tool to determine where your tops are. Use the Stop printing function in the Other tab, enter in 3x layer the layer thickness below a top layer(i.e. If your top is at 10mm and your layer height is .15, you would enter in 9.55mm. Create a new profile with 1 wall and use the Start Printing box to start at the point where your last profile stopped(9.55mm). Also your 3 wall profile doesn't need a top or you will have 4 solid layers in a row. You can repeat this process if your object has multiple Top areas.