STL is a series of triangles that mesh together to give the appearance of a solid body. Each side of the triangle is a line
Mastercam has taken those lines, and because it was windowed, tries to find an offset direction, chaining joins, and other possiblities . It can be a real mess to figure out.
Think about a cube, each face has 2 triangles, and 5 lines make up the face. ( each line can have a choice of which direction it is going)
Mastercam users make that chioce by "chaining" entities...when "window" an area is used, there is no decision made for this direction, and Mastercam is trying to think of all the choices it has to make.