Please explain how the popularity points is calculated. I saw my points fluctuate between 5 and 12 and today it is seven. Why is that? Also, I have 71 unique visitors in my blog, don't I get points for that. Thanks.

Your points fluctuate because so does your standing relatively to the competitors. The number of unique visitors is not a threshold that guarantees you a certain number of popularity points. It is a figure, which defines your rank amongst other competitors. Popularity points are ultimately awarded basing on this rank. That is why they are subject to fluctuations.