Had a similar issue happen to me a few months ago. My usage was way under according to IOS Cellular Data Usage AND a Third Party APP that measure my Cellular data usage. However, my Cellular provider (Rogers - prepaid) says I was over the limit. Called them and spend like 1/2 hour on the phone with customer service. All the time on the phone with customer service, I remained very calm and polite. I was able to get her to check the logs for big chunks of data and she was able to go through almost my entire month of usage and found no huge data consumption. I then proceeded with a little Math and we concluded with my usage average out that it was impossible to go over the limit. That's where she suspected something was wrong with their system and checked further. Turns out that there was a glitch in their system and my account was incorrectly flagged as over the limit.
Having a third party app (I use Traffic Monitor with Speed Test) helped by verifying that the IOS Cellular Data usage was indeed correct. I can see maybe the IOS Data usage and your Cellular providers differing by a few MB's but 200MB is a huge difference. You should call your provider to verify their logs.