I was surprised to see how strange my feed stats is when I logined my Feedburner account today. (I haven't logged into my feedburnner account for a long time) Anyone can help me to explain how this happen happens? Thanks.
Those are probably how many people read your feed on a given day. It's not total number of subscribers, but how many actually read the feed for that day. So it fluctuates, especially if you are posting once every three days (which is what the graph leads me to believe.)
Yep, feedburner stats seem to be roughly "who read your feed within (roughly) 24 hours". So if your site is not updated daily... you'll get stats like that.