In this study, which I am only assuming is the one you're quoting, and others, they classified counts of <100,000 sperm/mL as azoospermic. So even those who were counted as sterile were not, in fact, sterile.
World Health Organization Task Force on Methods for the Regulation of Male Fertility. Contraceptive efficacy of testosterone-induced azoospermia and oligozoospermia in normal men.