Why RSS readers number change every day
The FeedBurner chicklet represents the number of people who read your blog with some kind of RSS reader in a 24 hour period. Subscribers is inferred from an analysis of the many different feed readers and aggregations (including RSS by Email) that retrieve this feed daily. Subscribers is not computed for browsers and bots that access the feed.
The number goes up and down because not everyone checks their RSS readers on a daily basis. The number generally goes down on weekends because people are out shopping/playing/doing whatever instead of being on the computer. A decrease in the number does not mean readers have unsubscribed from your feed. It just mean not as many readers accessed the feed that day.
This post is dedicated to all RSS lovers who look to subscription numbers every day. I think this post will calm many of them!