Why do some people wear sweaters even if its 100 degrees out?

Today I saw a young woman walking with her friends somewhere. It’s 106 degress today. She was wearing a button up sweater (unbuttoned though).

Are there really people out there who are chilly in 100 degree weather? Or is something else going on with her? I’ve seen this before in people. I’ve seen it in the homeless, but that’s just mental illness, right? What’s going on with the seemingly healthy people who wear an extra layer of clothes when its hot out? Does she think she’s fat? Is that it? Even if that was it, the sweater doesn’t cover that up, it highlights it. She was attractive and almost, but not quite, chubby. It usually is women I see who do this… the only males who do it that I’ve seen, are homeless.
Her face was not covered up, so it has nothing to do with protecting her skin.

So what are the reasons a person would wear a sweater on a 100 degree day?