It's not 'the same populations' -- the populations change over time. Let us say that you have a new disease. 10% of the population is completely unprepared for this. They are going to get sick with it at the slightest exposure. 10% of the population had something very similar last year. You could sit them beside sick people all day long, give them the maximum exposure possible and they absolutely never will get sick. 80% of people are neither. So here we have a population where 90% are at risk for disease, and 10% are near certain to get it.

Let the disease go around infecting people for a while, until all of the 10% have become sick and recovered. (This is my imaginary disease, so I will make it one that doesn't kill anybody.)

Now when you look at the population, you see that the 10% that had something very similar last year still aren't getting sick. The 10% that were at great risk for infection, well they all got sick and now they are protected too from the illness they just had. They won't be getting it again. And those 80% in the middle? Most of them haven't been sick but some have. Let us say 5%. They are protected too.

Now we have a population where 75% are at risk. The virus is the same. The population has changed, by having members getting sick and becoming immune.