If `USER_SYNC_ATTRIBUTE=email` why is `OKTA_USERNAME_ATTRIBUTE` required/used?

[seanorama]

In the Okta provider the sample .env states:

## The attribute which corresponds to the GitHub Username
## NOTE: This cannot be an email address
OKTA_USERNAME_ATTRIBUTE=github_username

If the user can be determined by email lookup from git, earlier in the .env:

## Attribute to compare users with
## username or email
USER_SYNC_ATTRIBUTE=email

Why is the git username needed, when you have the email and ability to look up the username from that?