Explanation: People from different countries can learn English and understand the daily communication needed to function normally. However, when it comes to expressions, slang and phrases that are characteristic of English, as well as any other language, it is necessary to get into the spirit of the language, i.e. to know something more than the language. It implies a way of life, a tradition, a slang that even varies from country to country within the English speaking area. In other words, more time should be spent in a country where English is spoken so that people can start joking in English and understand slang and use it.
Basically what happened was the population went through a population bottleneck in the 1800s which then resulted in the only way for future offspring was a lot of positive assortative mating which results in fewer genetic variation(similar to the founder effect for the second part)