"Yewber" would also be incorrect. The relevant sound simply doesn't exist in English and is difficult for Americans to pronounce, so we don't use it. ü and u sound virtually identical to us.
It's like how Japanese people saying "basu" instead of "bus" aren't incorrect, they're simply adapting a loan word for their language's phonetics.
Could it also mean "to the north of"? If I was looking at the map or globe with my kids and they asked where various countries were I might say that Mexico was under USA and Canada was over it, or that Papua New Guinea was up from Australia... I realise this would be a specific conversational context rather than formal language, I'm just wondering if it would be natural to use it in this way.