Congratulations on the job offer! Companies and organizations have their own set practices by which they extend job offers, some do so through the hiring manager while others communicate offers through an HR partner. It's not uncommon for the person who has worked most closely with you through the interview process to extend the offer. You may see the Hiring Manager's HR Partner step into the offer process as you negotiate salary, or other components of the offer. Additionally, if you had questions about the benefits offered the Hiring manager may refer you to their HR partner.