I have found that copy and pasting is very finicky in the editor. We seem to be having a problem with consistency of our knowledge base articles and we copy a lot of text into the editor. It is very difficult to get all the text to a uniform font, size and color. Even if you are to get all the font to a "black" color, the shades of black still seem to be different between what is typed and what is copied/pasted in.
Copy and paste can be challenging with the rich text editor. Which version are you using, and which application are you copying from? I can add a note to any related known issues with the additional information, and possibly help with some ideas depending on the situation.