Yeah, the population for some of these are way off. Singapore as a country has a population of 5.6 million, and Durban is nowhere near either. Also Dhaka is in Bangladesh, not Senegal.
They are not different sources but different lists that show different statistics. The quiz description still says cities with over 10 million but the quiz does not reflect this as some go as low as 3 million. If you want to use 'city proper' populations you should state this in the description.
https://en.wikipedia.org/wiki/Megacity
You are using this list:
https://en.wikipedia.org/wiki/List_of_cities_proper_by_population