This is pretty common for both sexes. I've noticed more IOI's coming unexpectedly from girls with guys AND when I'm with a girl I get a lot more IOI's from single and paired off girls.
It's the fact that you're preselected that tou get these IOI's. When you're not rich, famous, or attractive, the best way to get a girl is get a girl.
Take it for what it is. People are hard wired to flirt. Girls feel more safe and self confident when they're with a guy. They're not "using" you go build their ego or anything, don't read too much into it.
Of course they feel safer when they're with a guy! That way IHAB is more believable. Girls are hard wired to flirt because they love attention. We, as men, care more about getting sex and relationships. Attention has little meaning to men. The end result is all.
Take it as a sign (or proof or a useful hallucination or whatever) that MORE GIRLS THAN YOU KNOW WANT to flirt with you, but only those that are confident and safe enough do so.
Again, if both sexes just flirt with each other and the girl rejects the guy in lieu of having a BF, that's just the girl getting some needed attention, and the guy didn't get sh*t. The end result is what matters. Flirting is just a small part of the puzzle. Flirt all you want, but if you're not getting these girls, you're not getting anywhere.
Then find those single girls and help them ease out of their shell, assuming they WANT TO but are just too shy. This is what having a STRONG FRAME means. Her not flirting with you doesn't mean she doesn't like you, it means she wants to and is too shy. HOLD THAT FRAME and keep it while initiating contact, and she'll slowly start to accept and REMEMBER that interaction through that frame.
Ice breaking. It takes time and her interest level matters a lot here.
You could say that waiting for clear and obvious IOIs is a sign of a WEAK FRAME, meaning you only talk to girls who give you permission first.
An obvious IOI is not obvious. It could just be a cry for attention. You need to take your own initiatives. A girl ain't gonna fall in your lap. Why do guys turn their aloofness and ignore women stats up to eleven and wonder why a HB9-10 isn't dating them a month later? Easy, because fortune favors the bold. Closed mouths don't get fed. But if you call her out (make a move) and she backs off, either:
1. She just wants an orbiter or friend
2. You miscalibrated your escalation
3. She has little to no desire/ interest in you
If you wanted to play with those flirty married girls, walk up when they're alone and say something like this:
"You know, it's really too bad you're married, because I'd really LOVE, to (pause look them up and down)..." and then walk away.
Likely you'll get a chuckle, but it all depends on her interest in you. Move on if she's lukewarm or lower interest.