Differential based on old backup. Oops!

Created a new backup scheme from scratch (see my other thread). When I check to see how it ran, I see it starts off with a diff file instead of a full. Huh? So I look at the backup itself, and it's based on a full from 2012 which is in a different directory, from an old computer, and a different version of Acronis. What? How could this even happen? I'm going to stop the backup routine because I certainly don't want Acronis to clean up and delete this old 2012 copy of a defunct computer that I was saving for a "just in case I need something" moment.

See this video:
http://screencast.com/t/f8czTBkSX
I show the the backup MWF 2014 and open the directory, see how it has only one file, a diff. Then I click the restore and see that the first version is from 2012! Huh, it's located on a different directory entirely even.

How did this happen, and how can I prevent it?