My bet is on one of

- A sequence number being accidentally used as the private key or

- The private key being generated securely but then accidentally truncated to 8 bits (e.g. writing a single char into a zeroed buffer when you should've memcpy'd from a char*).

The sequence number idea is supported by the fact that two of the G3B private keys are consecutive numbers. I ran a quick Monte Carlo simulation and if I got it right, sampling 8 keys from [0,255] uniformly at random (as in the truncation scenario), you'd expect them to contain 2 consecutive numbers only about 18% of the time.