Zinc decreases DHT by inhibiting (reducing or stopping) 5 alpha-reductase, an enzyme which is responsible for converting Testosterone (and other hormones as far as I know, but the science on that is not as well-researched from what I've been able to find) into the more potent DHT. DHT = bad testosterone, DHT causes prostate issues in men and also hair loss.
That study is an anomaly. You know how 9 in 10 Dentists recommend Colgate toothpaste? This study is that 10th dentist.
Ok, that's a bad example because Colgate sucks. But you get my point.