You might have had a large % of positives wiped from say January 2014, which has bought it down.. If your only negs or neutrals were recent, it will drop further as you lose more positives each month

I think..

Yes agree with other posters.

If you want to prevent further drop of percentage rate, based on the timing of your single negative,  you may need to sell more goods that result in postive feedback only, it depends on the date that the single negative feedback was received and the total number of positives that will also be included in the annual count.

Or you could buy stuff too, all positives count towards the percentage.

It's only a matter of 10 feedbacks or so making the difference between whether your % gets rounded up to 99.9 or down to 99.8. Doing the maths:

666 divided by 667 = 99.85007%

656 divided by 657 = 99.94779%

Anyway, yeah just buy 10 more low value things as the other poppy has suggested if you want to see your % go higher.