I always assumed that that was a consequence of their demographics being extremely skewed towards people in their twenties and thirties who immigrated with the intention of starting a family, with relatively few older people and almost no children.
The Aurora growth rates for small colonies appear to be mathematically impossible if we assume otherwise. For a population growing at a constant 10% per annum, the cohort aged n years must be 10% larger than the cohort aged n+1 years, and so on - we can construct a sample population pyramid using this, and determine the required fertility rate using the following approximation:
(1/(1 - imr)) * power[gr, anm] + power[gr, (anm - le)] = sum(n=1 to n=x) {cf x power[gr, (-tbp * (fr-1))] }
where imr = infant mortality rate ~ 0, gr = growth rate = 1.1 (10%pa), anm = age of new mothers, le = life expectancy ~ 90, cf = childbearing fraction ~ 0.5, tbp = time between pregancies, fr = fertility rate
If we assume that the colonists are having children one year apart (this is extremely short, realistically it'll be two years or more), then we end up with fr = 6.73 and nma = 7, i.e., the average age of new mothers is seven. Trying to bring nma up to 18 takes fr to 21 and tbp to 0.33, i.e., everyone is having triplets every year for seven years in a row.