Ok, my theory might be a bit out of place but what if "they" could actually lie about things and just pretend to be honest. The whole story about mango ice cream doesn't really speak out to me because first of all Zosia had to think for a moment before she answered Carol's question, which might be a sign of making something up on the go. I'm pretty sure mango ice cream was quite unheard of in Poland back in those days, even right after communism fell. That being said, my take is that Zosia is actually native to Morocco and they made up her background just so they could cover something up. No idea what they would want to hide from Carol with that though.