Sometimes data between smartapp & your carrier's App may not match due to various reasons including:
1. You have not entered your plan details correctly in smartapp
2. Your carrier App doesn't update data in real time. Some carrier apps update data usage at specific intervals instead of real time unlike smartapp
3. If you are on family plan then the data may not match
4. smartapp was disabled by you which could result in data mismatch. We recommend that you never kill the app instance to accurtely track the data