Ha, I guess there probably was a misunderstanding there. Karen was giving you the precise sound of 去 in Taiwanese Hokkien language (spoken by roughly 70% of Taiwanese people), not Mandarin Chinese language (spoken by roughly 100% Taiwanese people). It's can be confusing for people to ask/answer questions under such a category named 'Chinese (Taiwanese)', but anway.
Your question/doubt is quite common for people who learn Mandarin Chinese through 'pinyin phonetic system' which has got many shortcomings itself, for example, the 'letter sounds' are not consistent.
1) bu, pu, mu, fu, du, tu, nu, lu, gu, ku, hu, zhu, chu, shu, ru
2) ju, qu, xu
The u in list 1) and the u in list 2) in fact sound differently.
The u in list 1) sounds just like 'oo' in English word 'book'.
The u in list 2) sounds just like a 'French u'--I find that you're learning French too and thus I give you this explanation.