Because Welsh u is not pronounced like in English :)
In the south, I believe it's similar to the "i" in English "sit" when short, or like the "ea" in English "seat" when long. So it sounds exactly like Welsh "i".
In the north, the sound is a bit different - a bit further back in the mouth. But still not like "oo" in English "boot" -- that would be "w" in Welsh!