I spent some time puzzling over that, too. Here’s what I think is going on …
It processes 200 of your Anatidae at a time, newest to oldest observed. Each time it finds one older than the years counted so far, it returns the difference in years between the one it found and the maximum year found so far (where the starting date is today).
Now, I think what happens next is those are getting accumulated by the main loop into a counter until they reach the goal of 10, but I must confess I broke off studying what the rest of the code does right there.
In my case, I had five observations, one each in 2022, 2021, 2020, 2018, and 2017. So 1 + 1 + 1 + 2 + 1 = 6, so I hadn’t yet reached the goal. The nice thing about counting them this way is seeing a cross section across the years of the different individuals I found, rather than just the oldest ones. But it shows as 50% there (5/10) which corresponds to the number of observations found, not the score which is only referred to internally to see if the goal has been reached yet. At least, without having read all of the code, that’s what I think is going on.