Let's say I have guest posting enabled (but this can also be reproduced with a deleted account). You make a post as a guest and John Doe likes your post. I visit the forum logged out. The news feed will say 'John Doe liked your post in the thread Y' to me, even though I did not author the post.
The conditional in the template is a simple user ID comparison. If the author user ID matches the visitor user ID, it assumes the content is yours. Normally that makes sense, but all guests have the user ID 0.