Meeting invites for executive assistant going to CEO's inbox

Good Morning,

One of our users, an executive assistant, has access to her CEO's calendar and is able to successfully manage that calendar. However, when a meeting invite is sent for the assistant, the invite goes to the CEO's inbox allowing her to accept invitations on behalf of the assistant. 

In exchange, the CEO is not setup with Delegate access to the assistant's mailbox. Why is this happening?

Thanks!