As far as I know, even updates are counted as download. So it really is just the raw "how often was your iq file downloaded to a device" number... I don't know of it is aggregated by user though...
The download number everyone sees for an app are unique downloads by user. If the same user downloads an app multiple times for the same or even different devices, it doesn't change. The charts for downloads is total down loads. If someone downloads an update to the app you'll see that counted. When you upload an update to your app you'll often see a spike. Like you see in the chart in the first post for March. I expect that the app was updated during that month.
The May number is low as we are only halfway through May.
By the way, if you update your app in the store, but don't see a spike in the charts (especially daily), that's a sign that while users download your app, they might have tried it and uninstalled it as they aren't getting updates for it. This can be a bit obscured if you update the store too often. Doing that also can mess up your apps ranking and where is shows in categories.
