Copy & Paste

 Did you by any chance publish the post by copy pasting from elsewhere? May be a Google doc/Microsoft Office Word or any other word processor for that matter or from any website?

That could possibly be the reason for this mess up.

Alright. In blogger, copying from word document/another web page doesn't work out quite great.

Follow instructions below,

• Go to blogger.com Post List > click Edit on Blog Posts that you copy pasted from else where.
  
• Select all text with issue and click on Remove Formatting available in the tool bar. Tx
  
• This will remove all the formatting in your blog post. Now try formatting as you please.
  

To avoid this, you might paste using Ctrl + Shift + V instead of Ctrl + V. I hope that helps.