Why is the mobile so much different then the desktop version?
Why does the mobile not have the extreme test, timed practice or streak freeze. It's also easier than the desktop version because there aren't as many questions. Yet now that they got rid of the duels the desktop version seems easier (why did they get rid of the duels?)
Duels was a new feature that was available on the iOS version only where you can play against another user, but it was recently removed again, probably because they noticed issues with it. It's possible that it will be come back at some point, in which case it would probably be in a slightly different (improved?) form.
This inconsistency is why Duolingo's mobile interface should be cross-platform and based on HTML, not platform specific inconsistent interfaces causing inconsistency, usually that xxx platform specific app does not support feature supported on yyyy platform. If there is enough inconsistency, Duolingo desktop and Duolingo on some mobile will become different, no longer just an optimization for smaller screens.