This is a common misunderstanding, one which I was fairly confused by until recently. "Fertility differs from fecundity, which is defined as the biological capacity to reproduce irrespective of intention for conception [...] In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed fecundity. While fertility can be measured, fecundity cannot be" https://en.wikipedia.org/wiki/Fertility