Claude Code only stores the last 30 days of session history on your machine. Anything older has already been deleted by Claude Code itself, before Nessie ever sees it - so on first sync, the imported history will start from roughly the past 30 days, even if you have been using Claude Code for much longer.
This is a limit of Claude Code, not Nessie. From the moment you connect, Nessie keeps every new Claude Code session permanently, so going forward your full history is preserved.