I was skeptical at first, but it could be explained by the implicit wording of the sign-off. The highest rated sign-off "thanks in advance" implies that you're

I was skeptical at first, but it could be explained by the implicit wording of the sign-off. The highest rated sign-off "thanks in advance" implies that you're thanking them for their future action, which is most likely a response to a question in your email. Whereas the lower rated sign-off's "kind regards" and "best regards" don't imply that you're expecting a response at all.