[dahart]

Yes, exactly - I agree with your opinion and was suspecting the same, that substituting additional devices may explain the results.

But if untracked devices are responsible, or simultaneous online usage by multiple household members is responsible, then doesn't that mean it's possible that online time is not an inferior good? Couldn't it even be opposite of that, that higher income households are actually online more than lower income, because they have more devices, and don't use the primary home device? It seems likely to me that the idea of a primary home device applies more to, and affects more people in lower income households.