Turnitin: I'm a plagiarizer?

Today was the first time I've used Turnitin. I submitted a paper I wrote to see how it would fair under Turnitin's scrutiny. In my paper, I cited my sources, paraphrased, and used quotation marks. Yet Turnitin said that 21% of my paper was similar to other works. When I looked at the passages marked as similar and compared them to the originals, I noticed a few things...

I had never used (or even seen) the sources/papers to which my paper had similarities. Many of them were student papers submitted to Turnitin.

If I quoted a passage and a writer in the Turnitin database had also cited the same passage, my passage was flagged even though I had used quotation marks and cited properly. So if another writer and I used the same source and both decided to quote the same section, I get flagged as a possible plagiarizer.

Sections of the citations on my reference list were flagged because other papers in the database contained those same reference citations.

On the similarity report, I can exclude quotations and bibliography. Doing that decreases my paper to 4% similarity.

I can see a lot of value to using Turnitin or a similar service. However, there would need to be substantial (and repeated) training for instructors who use it so that they can accurately interpret the similarity reports. I'd hate to have failed my paper because 21% of it was similar to other works (including reference pages).