Slightly off topic: Why does Chrome, when the registration page includes both email and username fields, only remember the email but then insert it into the us

Slightly off topic: Why does Chrome, when the registration page includes both email and username fields, only remember the email but then insert it into the username field when you attempt to log in? I know some sites let you use the two interchangeably to login, but doesn't this seem like a silly assumption on Chrome's part? Why not remember both, and insert the username OR the email depending on what the field is called?