Yeah, those of us who have been on LTE for a while knew that this would be an issue with those switching from 3G. On my pre-LTE phones, I very rarely ever went over 2GB/mo. I've used several LTE phones now (Galaxy Nexus, GS3, and now the iPhone 5) and my typical use is around 5GB/mo and I'm on WIFI most of the time. However, I do some audio streaming and I download apps and updates over LTE at times. And I enjoy watching some Youtube videos when I'm out and about.
It's sad that we have these fast networks now but you really can't do things on them that you'd now able to do more easily. Correction: you can still do them, but expect to pay more for your data!
That's why Verizon will have to pry my Unlimited data plan off of my cold dead body. I just hope they don't start throttling like they do on AT&T.
Anyway, I read about a data leak issue when the phone is on WIFI and it still uses cell data. If that's they case, then I'd be gathering evidence and presenting it to your carrier if you go over your allotment. I don't think it's clear that this is happening for sure, or if it is, how much data is actually getting through.