You could have it in a significantly warmer area than I have mine in; your chicks could just naturally be hardier, or you might have a brooder that reflects the warmth back into the middle of the brooder. There are a lot of factors.
Hmmm. That's ringing alarm bells in my head. Some hatcheries will include a "Free" straight-run bird. Got any pictures? If she's a he, best to know now, rather than later.
My first suspicion would still be coccidiosis. The amount of amprollium in most medicated feed is enough to keep a low level low, but if the count raises for any reason, medicated feed often won't get it back down. Do you have pictures of this poop? Does she act cold? Are her wings weirdly held out?