I happen to be on good terms with the guy who supplied much of that info.
This got me:
These findings imply an "expected" hymenal opening size of 6 mm for someone JBR's age; her actual opening size, 1 cm, placed her in the mid-range of sizes observed in this study among six-year olds known to have been abused (see Fig. 3 of the study). 99% specific means that using this reference standard (1 mm per year of age), 99% of those with hymenal opening sizes above this standard were actually abused (i.e., the test is 99% accurate in identifying such individuals: only 1 percent are incorrectly labelled as abused). 79% sensitive means that 21% of victims of abuse were missed using this standard.