Why are Received and X-Received different?

What is the difference between X-Received and Received in email-header?

Received is a header defined in the standard while X-Received is a non-standard header added by some user-agents or mail transfer agent like the google mail SMTP server.

http://security.stackexchange.com/questions/103563/what-is-the-difference-between-x-received-and-received-in-email-header