This is likely caused by copying and pasting text from an outside source (MS Word, a web page, etc) into WordPress.
When you copy and paste from Microsoft Word into WordPress, MS Word brings its internal formatting – fonts, sizes, alignment, and much more – with it. These styles are embedded in the text itself and they tend to override the styles set up in WordPress. Your content may look normal on the visual editor screen, but hidden behind it is a lot of extra formatting.
To copy and paste content from an outside source:
- Copy your content.
- Click on the Paste as Plain Text button from the content editing box toolbar.
- Paste text into the text box.
- Click Insert.
- Format your text using the editing panel.