Was reading about Blockchain on Wikipedia to know more historically about it but I stumbled upon something new to Me, which was about the orphan block, which are not included on the main chain of the blockchain. During my research I discovered it's also referred to as stale block then later found out that there is a difference between them. Please what is the difference?

Also, according to the image above, why is the orphan block still added to the blockchain though is not on main chain of the blockchain?