Help me understand auto admits

UT Austin doesn’t have rolling admission, but why do auto admits get admission notification so much earlier? Is December 1 deadline only applies to OSS applicants? Can I assume that that by Dec 1, UT has already filled 75% of the admission with auto admits? Someone please help me understand this. Thanks.

Auto admits get notification that they are admitted but they do not find out about their major right away. Many don’t get their major and they choose not to attend.

The December 1 deadline applies to everyone. If an auto admit submits on December 2, they don’t get in.

My DD was an auto admit to UT Austin this year. She submitted her application in mid October 2015, and her status changed to auto admitted not too long after that. However, she didn’t get notification that she was admitted into her first choice major until late in January 2016. She enrolled officially in early Feb. This year’s auto admission is top 7% for the Class of 2021.