Why Rss Feed Count Decreased subscribers

Why Rss Feed Count Decreased subscribers?i.e why no of subscibers count on feedburner feed count decrease and increase every day.Thats good question asked by one rss reader of mine?How Feed burner calculates the subscribers count on its feed widget?It total accounts for the e-mail subscribers and no of hits given by your rss readers daily.i.e No Of Daily Subscribers=E-mail subscribers + Hits To