How reliable are the iOS data usage stats?
This is the extent of Apple's documentation on cellular data usage:


  Go to Settings > Cellular Data > Usage to view your usage. Contact
  your carrier directly if you have questions about your cellular data
  usage.


It doesn't sound like this is going to be very helpful. There are also threads online, such as this one, that seem to indicate that iOS6 may be using cellular data while connected to Wi-Fi and misreporting it.