Yep, it's a funny phenomenon, but I'll tell you from what I've seen, it's the same thing: women get more excited/intrigued watching a girl behave very flirty with you than they do watching one act shy around you.
I suspect the reason is they're erring on the side of not missing a genuine signal from another woman here; women will be genuinely very flirty with you if they're drunk and horny, for instance (though you typically won't have very long to escalate in that case before the window closes).
I've had pickups where the preselection I had prior to meeting the girl I pulled was essentially "fake"; the girl I talked to liked me as a person, and was fun and flirty, but there wasn't much or any sexual or romantic interest. And then she'd bounce off or I'd peel off of her, and the next woman I'd run into was ready to jump my bones.
Another factor playing into it is probably information loss in noisy or crowded venues or even if she's just far away - all she can really see is some girl is acting animatedly toward you, she can't get a good enough feel for what the intent behind it is. So she just assumes you're hot stuff.
Because women are so competitive with each other, they seem to be wired to err on the side of not missing an opportunity for a desirable man, same way men are wired to err on the side of interpreting interest from women as attraction even when it may only be friendliness.