I took the September ACT and had my scores sent to the University of Tennessee (which I'd already applied to). When I was checking my application a few weeks after the ACT, though, I noticed that scores were posted for the September test, even though I hadn't gotten the scores yet. Is this an error? I'm confused.
Yes. Because most colleges receive scores electronically, it's possible for them to get scores before you if you're receiving a paper score report.
As this article states, it usually takes the ACT about 2 weeks to score the multiple choice section and another two weeks to score the Writing section (although it's been taking a little longer with the new ACT Writing). Colleges then have to receive the scores and enter them into your application. If you are receiving a paper report through State and District, School, or DANTES Testing, you’ll only be able to see your scores online after getting your printed score report in the mail, which means that if a college has elected to receive electronic score reports (as most do), then it's entirely possible for your score to get to a school before you've gotten your score report.
I think it's probably pretty unusual for a school to not only receive your score before you do, but to enter it into your application so promptly. Since you applied so far in advance of the University of Tennessee November 1st priority deadline, however, my guess would be that there aren't a lot of scores coming in, and so the admissions officers were able to update your application speedily. So if you do take the ACT through State and District, School, or DANTES Testing and send those scores to a college, it's always worth it to check your application on the college site and see if they have your ACT scores already.