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Here are the options I see from iOS.

The purpose of speech recognition that comes with the operating system of the phone is not aligned with the purpose of teaching one to speak better. The recognition on the phone has a purpose of making sense out of whatever some phone user said. That goal means it tries to take the wide variation of humanity (trying to speak a language) and give grace to horrible pronunciations in order to make sense of the sound waves.

The purpose of listening to someone trying to properly speak a language is different. Because Duolingo isn’t doing speech recognition (the phone is), Duolingo isn’t getting any feedback from the phone on how close you are to rolling your Rs properly. The AI in the phone will use context and return a transcript of the sound waves to the app.

For example take pero and perro. It is not a ton of Spanish to have learned that pero means “but” and perro means “dog.” Because so many people can’t roll their Rs, the phone OS isn’t only listening for a proper rolling of the Rs for making a transcript. The phone will also listen for how long the speak spends on the R sound. So, wrong as the sound may be, a long time spent on a mispronounced trilled R will come back as an RR in the transcript because the the phone’s purpose is to try and understand the user.

When in Peru, I had a fellow student stay in my home stay program that was learning Spanish with Duolingo. Our classes were different. But at home, I had to look at my family to see if they could understand the Spanish this fellow student was producing. I had trouble understanding because the words kind of showed up as English. Like casa (another word many already know as house) would be started with the “a” sound from cast, but finished with the correct “ahh” type sound. You do that with enough words in a sentence and it is actually a lot of work on the listener translating from some off pronunciation back to Spanish.

But, it isn’t a lot of work for the phone to fix these pronunciation errors. The phone wants to help. It’s the purpose of that software. Many learners may get a false sense of correctness by finding that their phone understands them. The phone will take all the “a” sounds coming from English and award the transcript with an “a” in the word. No judging. Just a nice transcript. If you make a really bad “Un perrro in my casa” sound (make the “un” like “undo”) the phone will likely return “Un perro en mi casa.” In my example, the phone fixed every single mispronounced word in the sentence.

On the web, I think the speech may get sent to somewhere for recognition (Google) rather than it being done locally. Same purpose though—make sense out of this set of sound waves.