From Billboards official year end single charts for each year.

Video explanation – This the year-end singles chart for US billboard.

Meaning just for that calendar year itself, so a big song that was released that year, but in September lets say, has much less chance of making it to the top of the year end chart than a song that was released right at the beginning of that year or even at the end of the previous year.

Its not about what the peak chart positions of songs were, or how big they are today etc.