Im not an expert, but i think it probably has to do with the way you pronounce the "r" from the back of your throat. It sounds like you'd use a lot of air like you do for the letter "h". In english, we say the letter 'r' from the middle-back of our throat rather than from the back like in German. However, if you were to say "hund" (dog), you would be making a "oo" sound but in the form of a breath (say it really slowly and see what i mean). If you say "rund" (round), the german "r" sound, your throat woule be much more open than with an 'h' sound. The word hund is literally pronounced the word und but with an exhale before it, but rund requires a more open throat and slight "r" sound. Again, im not an expert, im just a normal Duolingo learner like the rest of us!