What does the equal sign mean in TortoiseMerge?

In TortoiseMerge document, the equal sign means :

A change has been undone by reverting to the original content of the line.

The panel shows the differences between Mines or Theirs and base, my questions are :

If something is removed, then minus sign can be used. If something is added, then add sign can be used. Why is the equal sign needed? What does the description "the original content of the line" mean?

Mike's answer is correct. To elaborate a bit, this is an artifact of the way subversion merges changes. It applies all changes that occurred between the merge source and destination, regardless of which changes you told it to merge, and then "undoes" any changes that you didn't specify.

Here's an example: I branch a file from my trunk at rev 1. I change line 100 in my trunk version and commit it as rev 2. Then I change line 200 and commit that as rev 3. If I then go to merge only rev 3 into my branch, I will see one of these equals symbols at line 100 because subversion merged rev 2, merged rev 3, and then un-merged rev 2. Since the user specified only to merge the changes in rev 3, the change in rev 2 is NOT included in the merge.