Exactly this, my former roommate used to flirt with me all of the time, but I knew she wasn't actually into me. It's just how she acts with her friends.

Meanwhile, a coworker that apparently was into me never acted as if she liked me as more than friends, so I treated her as a friend. I had to find out much later from a mutual friend after she assumed I must not have been interested.

For some people, the "obvious signs" aren't signs at all. Other people think they're telegraphing strongly enough to be seen, but aren't.