Weird edge on flat plane I have a cylinder with a hole in it.
The top face of it should be a flat plane but it isn't,
even if there are no edges on it which could cause the unevenness.

I inserted an additional edge (the marked one) so that you could see what I mean.


Why isn't the face a flat plane and how can I make it flat?
I tried to delete the top face and replace it with "fill hole" but that is not possible.
I found a way to make it flat.

There is a second fill option (not fill holes) with ALT + F or under Mesh -> Faces -> Fill (with tries)

With this it worked.

EDIT:
There is a third fill option (GridFill) with Ctrl + F (with quads)

Thanks to Mr Zak in the comments.