Here's one way to look at it--an .iso is simply an image of what's on the disc. It's exactly the same size as the disc. However, what happens if you try to write it to the disc as a file?

In that case, the burner software tries to put the file knoppix_image_file.iso on the disc, and ALSO some extra file system information which says "There is one file on this disc and it's name is knoppix_image_file.ico and it's located in sectors XX to YY."

This file system information is relatively small, but it still requires some extra space. Since the .iso image is exactly the same size as the disc, even this small amount of extra space puts it over the top. It won't fit!