What's the deal with this??

I been talking to this girl in a class setting for like 8 months now. Somewhere in the first 2 months she mentioned her bf, but it seemed reactionary (like out of jealousy) because I mentioned my gf. I didn't show much interest in her, but she somewhat pursued me at the time.

Since then she has never mentioned him again. Just that 1 time. And we always talk and ask about each others weekend. We talk about life, intimate topics, and she seems curious about me. She never brings him up.

So I assumed they were broken up. I was having problems with my gf 1 month ago (we're on and off a lot), so I asked this girl for her number. I finally pursued her back. She gave it to me without hesitation. But she acts avoidant through text. Doesn't respond much; doesn't starts convos. Whereas in person, she's seems into me (the smiling, laughing so easily, asking about my life, sharing about herself).

So I look up her social media and see that she's still with her bf! Despite giving me her number, and allowing me to pursue her..

What's up with that? She obviously must enjoy the attention, and it seems like she's on the fence about me sometimes. When I was with my gf she even hinted that maybe she'd go out with me if my gf wasn't in the picture (my gf is back in the picture now, somewhat). But this girl has a bf!

This isn't the first time this happened to me. Years ago when I was on and off with my gf again, another girl told me she'd like to go out with me if it wasn't for my gf (and gave me her number), but she had a bf at the time too! And never told me about him! I found out through social media stalking.

What's with this behavior? You want me to leave my gf, but you have a bf? I'm thinking they want to monkey branch but need more certainty/security that I'll be available. Or maybe they just like the attention.