Your phone and the network both count your data, and they will never agree exactly. Neither is broken.
They measure at different points
Your phone counts what the apps on it send and receive. The network counts what actually crossed it, which includes protocol overhead the phone does not attribute to any app. The network figure is the one your plan is billed against.
They cover different periods
Your phone's counter runs until you reset it manually, so unless you reset it when the plan started it is counting a different window than we are.
Reporting is not instant
Usage is reported to us by the network in batches, so a large download can take a while to show. Right after heavy use, our number is usually the lower of the two.
Which one to trust
Ours, for deciding whether you have data left. Your phone's, for a rough sense of which app is using it. See "Check how much data you have left" and "Top up a plan that has run out".
When the gap is too large to explain
If the difference is dramatic rather than a few percent, tell us. Send what the app shows, what your phone shows, and roughly what you have been doing. A genuinely wrong figure is something we want to find.