[SA-exim] Hits get minus value

[Pradeeper]

Hi All

I get some small number of SPAM mails with minus hits value.
Headers are like this...

Mime-Version:  1.0
X-SA-Exim-Connect-IP:  200.181.105.52
X-SA-Exim-Mail-From:  harriherman at netscape.net
Subject: You Won
X-Spam-Checker-Version:  SpamAssassin 2.63 (2004-01-11) on Trinity
X-Spam-Level:  
X-Spam-Status:  No, hits=-1.3 required=3.0
tests=BAYES_00,NIGERIAN_BODY1, NIGERIAN_BODY2,NO_REAL_NAME,US_DOLLARS_3
autolearn=no version=2.63
X-SA-Exim-Version:  4.0 (built Sat, 24 Apr 2004 12:31:30 +0200)
X-SA-Exim-Scanned:  Yes (on Trinity)

......

Why is this? Is there any thing I can do about it?

Thanks!

Pradeeper