Hi sushiljain,
Welcome to the AGL Community!
It sounds like you're being billed monthly. If this is the case, then your meter readings will be estimated unless we're provided a meter reading by yourself. As is the nature of estimations, they're not always going to be 100% correct however we can revise this retrospectively as it sounds like we've done in your case.
We understand that incorrect bills can be frustrating no matter the reason, so to avoid this I encourage you to either revert to quarterly billing so that your bill is generated every three months after your meter is read, or to enter your own meter readings online (using your AGL My Account or app) 2-3 days before your bill is due to be generated.
Kind regards,
Jayden