I have a dispute with my carrier such that my bills for the past 6 months list 10GB more usage than listed by my device's (iphone 5 running ios 6) data usage stats. They have told me that they can't do anything because they can't know how reliable the data usage stats are on the device. Are there any documents to which I might refer them that pertain to this question?
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.