Which beta version are you talking about? There were technically 6 releases of iOS 7 while in beta before it hit GM (though one was a security fix).
That said, are you on the last beta before GM? If so, like rockyroad said, it could be that you had less power than you thought. Are you using the phone when it happens? I've noticed when I get the 10% warning while playing music, texting, talking on the phone, or browsing the web that 10% goes by in a flash.
This is on a 4S though running the last beta before GM.