Issue title: a silly question
 Issue body: why most of new networks are fail and only a few are pass?
 Comments: 
 Comment 0: Not all training makes the previous network 35 Elo stronger. Most does not, in fact.
{'source': 'GithubIssues', 'repo_name_with_owner': 'gcp/leela-zero', 'issue_id': 308544758, 'repo_stars': 3142.0}