Nope, the reason isn't grammatical (well, not entirely).
Russian lacks articles, the main effect of this on Duolingo being that you need many more sentences in English as there are multiplicative additions through nouns.
So with one noun you may need 3 base sentences, two nouns means multiplying by another 3, having different verbs means even more such as "eats" vs "is eating".
There is just so many translations that can be correct that it isn't possible to catch them all upon course creation. They must rely on people to report when they are missing one so that they can add it