It is true that human population projections are subject to change due to fluctuations in global fertility and mortality rates.
Although people can predict to a certain point what the future population will be like, they cannot predict all of the circumstances in which the population will be changed. They have to take into account mortality rates of people, as well as fertility levels in the future, which is impossible, which means that there projections aren't set in stone - they are likely to change quite a lot.