how can I know real data usage

I am always wondering how ATT calculate the data usage, I am not a crazy 4G data user, just for email and gps, a few time a month, so I use 300 mb plan, as usual, I only use the data less than half of 300 mb in most month. however, every time I found, if ATT send me a notice of usage over 65% (when I am in travelling), then the usage recording get very strange, I means suddenly become very fast the usage increasing, even I do not use any 4g data. Yesterday, I turn off my cellular data after 3pm, but this morning, I found in my account there are still having usage recorded: 3.5mb,9.3mb and 7.5mb from  5pm to 9pm yesterday, and the total usage has closed to 298mb of 300mb, actually my cellular data was totally in off!

Another strange thing is my cell phone (iphone 4s) has a cellular network data record, which shows only used 236mb during this month (including in and out).

I am really confused what happened?