Pure speculation: Is it possible that Comey did not want to disparage the NY FBI office and so pretended that the decision to issue the Hillary letter was his alone? Giuliani was certainly hinting at something before the letter was released and I wouldn’t be surprised that, if Comey hadn’t released his letter, the NY office would have would have claimed there was a coverup. The press would have jumped on those allegations and made them the main story for the few days remaining in the campaign.