We can't answer that as it depends. It can depend on your target market, the days in which it fluctuates, the genre of the game, even the different countries it fluctuates in all and more are a factor of these reasons...
For example off top of my head say your game was targeted at school kids in a church going country (as a majority ofc) and your days were sat,sun,mon. now assuming they still go to church on sunday mayb downloads were high on a saturday as they were all off, sun they discuss it in church and mon their friends try it, try not to look at the ups and downs (your go insane) instead try looking at trends ie over a month it has drops by 20 points theres something wrong or could simply be market saturation.
What you essentially have is a product in a fierce market and all the rules of trends of products will take effect. Day by day is not a good measure as there is too many factors involved.