i looked at the page but i don't get that..
Do you mean that you don't understand ?
That's quite simple : from 100k pop, your pop growth slows down, and does so up to the point of no growth at all or a small decrease ... that's when it will stabilize.
Medecine level increase the threshold at which the pop will max, but doesn't change anything about the process.
Granaries increase your pop growth up to 100k pop, but far less so after it.
As in the rules, each terrain has a "natural" pop capacity, with plains welcoming the largest populations, and mountains the least. Meaning that 1) mountains will reach their max population earlier than plains, and 2) population growth will decrease faster than with plains
Have fun,