The results are compatible with "risk homeostasis" or "risk compensation" theory. People who use unit tests have an increased sense of safety, so are more comfortable living with less-safe designs and code-bases.

"Risk compensation is a theory which suggests that people typically adjust their behavior in response to the perceived level of risk, becoming more careful where they sense greater risk and less careful if they feel more protected. Although usually small in comparison to the fundamental benefits of safety interventions, it may result in a lower net benefit than expected" -- https://en.wikipedia.org/wiki/Risk_compensation

http://injuryprevention.bmj.com/content/4/2/89