This is what I love about Duolingo. Funny sentences that make zero sense, but are still helpful in learning the language
You can see the differences clearly in Formal Standard Arabic.
1) A Kareem's dog: كلبُ كريمٍ - kalbu kariimin. This possessive structure is called as 2iDaafa الإضافة. We can see both words has the differences at their ending sounds, -u vs -in.
2) A generous dog: كلبٌ كريمٌ - kalbun kariimun. كريم here has the same ending sound (-un) with كلب because the previous is the attributive adjective of the later.
"Kareem" (Arabic) could have been wrongly transliterated to "cream". Happens when Google Translate does not know the actual translation and just ends up transliterating it.
By the way, this tool heavily depends on inputs and corrections from users. So it could be wrong. Whenever you see it wrong, just flag it, or if possible provide a correction, so the translation gets reviewed ;)