I would say there is probly a mixture of two reasons for this. One, she wants to avoid laying in bed by herself waiting to go to sleep... probly due to anxiety if she's anything like I was at her age. I had, and still have at times, a very bad way of "thinking about the negative"... when I was laying in bed at night, I would constantly think about and even picture, our house being broke into and us being killed, the house starting on fire and someone dying, etc. All the bad things. To the point that if I didn't find a reason to be up, I would lay there and cry myself to sleep. You think you would know if your child was doing this, but trust me, my parents... who were good parents and paid attention most of the time, etc... had no idea. I just told my mom a few years ago and I'm 28! That wasn't until I virtually had to tell her because my son was born and it all started again, but with my infant son.
Two, she wants attention. I also have 4 kids. If they're all going to bed at the same time, your child knows that if she gets up at that point, she doens't have to compete with anyone for attention... no being interupted, you're not doing something for someone else, etc.
My advice would be to make it a nightly routine to spend 10-15 minute with her when she goes to bed... just lay with her, get her calm and thinking positive, and give her some one on one attention. Don't get me wrong, I'm not saying you starve your kids for affection or attention. But it might help.
On top of that, make it VERY clear that no one is allowed up after a certian point. I give my kids a 15 min. and 5 min. warning before quiet time starts... at 15 min it's time to go get their pjs on... at 5 min, they better have bathroom trips, water trips, etc. done and be heading to bed and they have no excuse to get up again.
Hope this helps.
P.S... my daughter is 11 and does the same thing if she's allowed. I thought the same thing too... "not out of an 11 year old"... my 7 year olds go to bed and I don't hear them again till morning!