Copying and Pasting content from rich text sources into WordPress does not work as you would expect.
We get this question all the time from clients who either copy content from other web pages, or from applications like Microsoft Word.
Put simply, it does not work as you might expect. Some styles are picked up from the source document when pasting into WordPress text editors. The result in unexpected formatting of the pasted content.
This video demonstrates the problem and shows you a way to work around this.