You are probably aware that CKEditor's textarea is basically an iframe. And you must have noticed that the width of the input text area exceeds the iframe's size on several occasions. A horizontal scrollbar appears, making you scroll to read the text on the right.
Here is an image of how it looks:
There is a very simple fix to resolve this horizontal scrollbar issue. Go to the CKEditor configuration page. Under "CSS" -> "Editor CSS", choose "Editor default CSS".
After you have made the changes, return to the CKEditor and you will not see horizontal scrollbars any more.