Matt,
Temperature ratings on sleeping bags are not really all they're cracked up to be. Call them "guidelines" if you will. From what I've been told, sleeping bag temperature ratings are meant to describe the maximum comfortable temperature conditions that a naked human body can sleep in.
Consider this: My basic sleeping bag is a high-loft down filled bag with a temperature rating of -7 seven degrees. I have slept comfortably with it in a tent at temperatures approaching -20 degrees celcius. All I had to do was wear a warm toque (woolen hat for those of you not accustomed to Canadian English) and a fleece jacket overtop of my standard base layer underwear
I hope this helps.
Adam