How can that be?
If you dig into the data there are a couple of big clues and my guess is that there are several building blocks which, when tied together point to the reason.
Apple sells mostly high end devices to people who are either looking specifically for a smartphone or looking to replicate something they've seen done on an iPhone elsewhere.
Android phones sell mostly at the low end and are generally bought as a replacement for a feature phone. In developing countries this also means a limited access to mobile data, which can be both expensive and slow.
If you look at the figures from various pieces of research you'll see how this translates into usage.
Android users spend more time making calls. Most high end smartphone users will tell you that calls are probably not one of their first five smartphone uses.
In all likelihood if you were to match a high end Android user and an iPhone user on overall activity you'd find that they were incredibly similar. However high end smartphones don't make up a large chunk of total Android sales, resulting in a far different picture when averaged across the whole community.
Is it a telling statistic? Probably not unless you're a developer, in which case its a justification for focusing development time in the iOS world, where you'll find a generally more receptive audience on average.