Over the weekend, Asymco‘s Horace Dediu took a closer look at IBM’s data, which he’s been tracking for the past three years. As the chart at right shows, more people were shopping by Android device two years ago than by iPad. By last year, the iPad had caught up. This year, iPad usage was more than double Android usage.

The iPhone is also outpacing Android phones, as Dediu is able to show with a second chart that combines IBM’s shopping data with ComScore sales numbers. It shows traffic per user growing on the iPhone and dropping on Android phones.

“One would expect that later adopters would engage less,” he writes. “But I find it surprising that U.S. Android users would behave so differently only two years after the platform began to be widely adopted. That pattern is not happening with iOS even after five years and certainly not with the iPad which is about as old as most Android brands.”

Dediu continues:

This I consider to be a paradox: Why is Android attracting late adopters (or at least late adopter behavior) when the market is still emergent? We’ve become accustomed to thinking that platforms that look similar are used in a similar fashion. But this is clearly not the case. The shopping data is only one proxy but there are others: developers and publishers have been reporting distinct differences in consumption on iOS vs. Android and, although anecdotal, the examples continue to pile up…

I’m not satisfied with the explanation that Android users are demographically different because the Android user pool is now so vast and because the most popular devices are not exactly cheap. There is something else at play. It might be explained by design considerations or by user experience flaws or integration but something is different.

As Dediu points out, user engagement is not a frivolous platform attribute. In fact, it’s critical for companies like Google (GOOG) and Amazon (AMZN) that make their money from the ecosystem (i.e. ads and online shopping, respectively) and not, as Apple does, from hardware sales.

You can follow the lively discussion sparked by Dediu’s analysis at The Android engagement paradox.