Generally, how DNA tests work is by selecting points in the DNA and comparing the two samples. The more points you compare, the more likely you have a true match. If you don’t compare enough points, or pick places in the DNA that are unlikely to have much variability across the population, you’ll get all matches on those points and say it’s a match. For paternity testing, you’re looking for ~50% matches.
Though, in this case, it does look like they were just making stuff up:
Richot said she was coached to ask women seeking prenatal paternity test kits about times in their menstrual cycles and the dates they had intercourse with different men — information that is useless for a DNA test.
Staff put the dates into an online ovulation calendar to narrow down the possible biological father, she said. Richot then entered the information into a form that went to Tenenbaum for signoff.
“[Tenenbaum] would always make a comment like: ‘It’s definitely this one [the biological father]. It’s this one, it’s got to be this one,’” said Richot.