Wondering the reason behind this 'massive' variation? Well, the answer is that numbers will go down when less number of people are checking your feeds in their desktop RSS readers (like
FeedDemon, Outlook 2007, RSS Bandit, etc).
In the case of stand-alone feed readers, that user has an application running on their computer which fetches the feed [from FeedBurner servers] repeatedly throughout the day...
The fluctuations are almost always due to people using stand-alone computers who don’t turn their computer on, or don’t load their feed reader on a given day. If their feed reader doesn’t ask for the feed that day, we don’t see them, and consequently don’t include them as a subscriber.
While FeedDemon or NetNewswire are also desktop based RSS readers, their subscription numbers are reported by NGO and hence don't influence the day-to-day fluctuation in Feedburner numbers. See Rick's comment below.