Why are there created points inside buildings when creating a grid even when this is not specified?

For this functionality to work, all buildings must be interpreted as closed polygons without the line of any building crossing itself (eight). The program automatically connects start- and end-point of the line, this can create problems if the line is not closed specifically.

Big buildings consisting of many small parts, may also create problems. Often there will be an empty space between the different parts, where points will be created although they in reality are inside the building.