Phone data monitoring software is usually not synced with account payment cycles. The counter often has to be manually reset at the beginning of each new billing cycle. And "30-day" plans are not quite the same thing as "monthly" plans.
Different softwares also track things different ways. There's no standards and there's lots of inconsistencies. Some count background data, some don't, some deliberately exclude certain apps/services from being tracked (sometimes including updates, syncs, speedtests, certain browsers, certain social media sites/apps, certain streaming sites/apps, and all sorts of background telemetry "services"), some are simply inaccurate. You can always install different apps if the ones built into your phone suck. And the only numbers which matter (which you pay for) are the ones reported in Self-Serve.
Finally, there's data overhead. Network-side machinery counts everything the network sends and receives. Client-side machinery (phones and devices) count everything they send and receive ... but this is only for user reference, it's completely ignored and irrelevant to the providers. There can be big discrepancies when your device does data in areas with low signal bars, when it moves around a lot (overlapping data sessions when there's handover between cell sites), when it has weak battery, when there's just a ton of other traffic (or noise) on the network which is beyond your control. Packets have to be sent and re-sent and checked and repeated more often, it all adds up and if you're in a fringe service area then the differences can be surprising.
SMS notifications were not sent when (account) data usage hit 75% and 95% thresholds?
