If the average score of a country is set at 100 how can that country score significantly lower than 100?

Hi, I’m sorry if this is a stupid question but I’ve just been following some Reddit discussions that confused me

Someone showed lists of different nations scores

Someone else said if lower iq individuals left a country the average would stay 100 because 100 is meant to represent the average.

So even though the average person would score better on an iq test the nations score wouldn’t change

Google was not very enlightening

Can anyone explain this to me?