Duolingo seems like they have separate teams for iOS, Android and Web.

Their iOS team seems to be the one who experiment the most with new features, and then they either get discarded or finally make it to Android, and then in a few years to the Web.

It is really annoying, as when I used Duolingo (stopped because of the new update in Jan), I would often use all three platforms, and have the worst experience.

The dev time on this must be very fast, given that it probably makes sense to focus on one platform first.

Also review times with Apple are consistently 24-48 hours, but Google Play review times are (sadly) not from what I’ve heard from developers. If the whole timescale is 2-4 weeks, that may be a deal breaker.

iOS is always first with new features in Duolingo. And from what I've heard from other sources, iOS users are more willing to pay $$$.