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It is a little confusing, but mathmatically there is no bug here. The confusion is caused by rounding. The message actually scored 6.950-6.999 (scores go to 3 decimal places), but the header only displays to a single decimal place. Therefore 6.9# (# is >= 5) rounds the whole value to 7.0.
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since there are only 6 stars.
See The rounding issue of the value has been changed for SpamAssassin 3.0; see also RoundingIssues