Do authors normally check their own papers for plagiarism? I was recently asked to review a paper (for a very reputable journal). In the cover letter, it says

"This paper has been checked with turnitin to ensure there is no similarity to other papers."

I haven't seen this before (although I have limited reviewing experience). Is this normal? Or does this indicate something strange about the author?
I've never checked any of my own papers for plagiarism. Should I?
EDIT: Also, why would you mention this in the cover letter?
I've never put such an assertion in one of my own cover letters, and would find it very strange to do so.


It's assumed that I should be behaving honestly and avoiding both plagiarism and self-plagiarism.  Saying: "really, I'm not plagiarizing!" just makes me wonder why the person felt they needed to say anything.
If I'm not behaving honestly, why wouldn't I lie about the results of plagiarism-checking too?


If I saw this as a reviewer, I would think it reflected an odd but almost certainly irrelevant thing about the authors, and then proceed to ignore it.