cellular-dataiosiphone
iPhone rounds cellular data usage reports to the nearest tenth of a Gigabyte (eg, 1.8GB) by default. Presumably the phone knows exactly how much data has been used and rounds the number for my convenience.
Is there some way to cajole (or even coerce) a more accurate cellular data usage estimate out of the phone?
iPhone 5s, iOS 7.1.2 and iOS 8.1
This is not a answer to your question, but some interesting information on what can eat your data plan. (even on standby mode) !
So if you used Google maps (and did not turn it off) on your Vacation, there is a possibility to consume 100 Hours x 22 Mb = 2200 Mb
As for monitoring probably this would suit you.
My Data Manager – Track your mobile data usage and save money
Description
My Data Manager is a popular app that will help you take control of your mobile data usage and save you money on your monthly phone bill. Use My Data Manager every day to track your data usage and ensure that you don’t go over your monthly data bill and incur unnecessary charges.
I had the same problem and the above fixes didn't help either. What ended up working was turning off the apps I wanted off, and then turning off cellular data altogether. Then moved back out of Cellular to the main Settings menu, then back to Cellular and my apps were still off. Then switched cellular data back on and the apps I wanted off stayed off. I also tried deleting and re-adding the offending apps and that worked too.
Best Answer
As it stands it shows the data with a (±) 50 Mb resolution.
You can see it as fine grained as per application data usage.
If you scroll down further (in Cellular Data usage), you will get an alphabetical list of all the apps have use cellular data.
If you scroll all the way down at the bottom, you will see a button called System Services. If you tap on that, it will show you how much cellular data you are using for things like Messaging Services, Push Notifications, Documents & Sync, iTunes Accounts, Siri, Voicemail, Software Updates, Find My iPhone, Diagnostics and Apple ID Services.