That it's likely a human-generated private key.

The standard deviations start from 0 to just over +-1, where +-1 (and above) mean that the key is definitely human-generated and 0 means it's definitely computer-generated.

None of the keys have std. deviation = 0 because you can never be quite sure whether a computer really did generate a key. But at low std. deviations you can be sure that the private key resembles as much as a random private key as possible.