OP solved it, closing

Have made same mistake.
Read description cearfully.

Don't want to spoiler, just read again:

"There are no fractions of people. At the end of each year, the population count is an integer: 252.8 people round down to 252 persons."

so if you get 1,99 in end of year, it should count as 1, not as 2.

Tell me, if that helped.