It's "qiTTatUkA" because "your cat" acts as 2al-mubtada2 (I don't know how to explain this in English but it is closer to "subject") and the cat's owner is male. If the cat's owner is female, then it's "qiTTatUki".
This is how the standard grammar is. (And, it seems Duo uses a dialect).
the word ‘Seth’ is not of Arabic origin. Yet the tradition of writing English names in arabic is to transpose consonants and vowels, in this case: s-ii-th
So my guess is the computer tries to Arabize it, treating the ii as a consonant (y) as in s-y-th
so add whatever vowel between the consonants. Suyith. Sayuth. Siyath. you name it :-D
But in Arabic it would actually just be pronounced: Seth