The download numbers for Chapter 29 are considerably less than those for Chapter 30. This is the first time I've posted two chapters at once, is this normal?
Yes, very normal.
If you check the download numbers for all the chapters you'll find there is no correlation between chapter download counts. Because the system doesn't keep track of who downloaded which chapter, each time a chapter is loaded it counts and if reloaded it counts too. So if a reader loads the chapter it counts as 1 download, if for whatever reason he closes the tab or his browser crashes, then he loads again, it counts as another 1 download.
The only reliable trends are that the first chapter has huge download count because of pre-loading, and the last chapter seems to always have more downloads than the couple of chapters before it because a lot of readers jump to the last chapter to vote or change their vote.