Why are some lines bolder than others


I can’t understand why some lines in this diagram are darker than others. If I select each line and delete it then I have a white space, so they aren’t double lines. Can anybody help?

Go View/Edge Style and untick Profiles.
The thick lines are showing you the edges that don’t divide two face.
Where you have those lines surrounding what you think is a face, the face is missing.
It can be helpful to use a non white background so you can see holes more easily.