@PPofEngrDr I do not really know! But I suspect that it may be the latter case you talked about. Here’s the school’s response on this topic:
https://www.admissions.illinois.edu/FAQ/admitted-freshman#Eight
"You were strongly considered for your first choice; however, we simply didn’t have room for all qualified applicants. You’re still considered waitlisted for your first choice. If space becomes available in this major later on, we’ll let you know even if you’ve declined your offer.
We strongly encourage you to stick with the major you were admitted into after enrolling at Illinois. If you’re interested in transferring to another program, we recommend you work with your academic advisor to learn the requirements and timeline needed to do so. If you were admitted into the Division of General Studies (DGS), you’ll work with an advisor to craft a unique schedule and declare a major before your junior year."