Because there is a population 'bulge' of younger people who haven't yet had children.
For a very simplified model, imagine at t=0 there were only 'x' people all of whom were twenty-years-old.
Run time forward to t=20 - some of the twenty-year-olds will have died, so the surviving population will be maybe 0.95x, but also some will have had kids, let's say the fertility rate is 1.0 children per woman, so the group as a whole will have had roughly 0.25x children. This means the population has grown to 1.2x despite lower than replacement fertility.
For a very simplified model, imagine at t=0 there were only 'x' people all of whom were twenty-years-old.
Run time forward to t=20 - some of the twenty-year-olds will have died, so the surviving population will be maybe 0.95x, but also some will have had kids, let's say the fertility rate is 1.0 children per woman, so the group as a whole will have had roughly 0.25x children. This means the population has grown to 1.2x despite lower than replacement fertility.