That's very true as this Emily already has three calves. Although she has had rather large birth intervals in the past; eight years and then six years. Astrid breeding rather early was possibly an accident, as it's rather unusual for a zoo to breed their females this often, Astrid did get pregnant again rather quickly.
Astrid produced her first calf at seven years old, which is usual for a captive female given the opportunity to reproduce. Half of the females that have conceived in New Zealand zoos conceived at this age for example.
Similarly, a birth interval of two years is not uncommon for young or prime age females. From what I've seen, birth intervals naturally increase to around three to four years as cows age, with any lounge e.g. eight years potentially indicative of estrus suppression etc.