The usage you see is based on CDRs. The legacy OCS only cuts a CDR at the very end of a data session. After the latest EPC core upgrades which tweaked the way the Diameter charging worked to better hold open idle sessions, our network is ironically now so good that you can go days without your session closing, and causing a CDR to be cut. This means you will often find you have "no" usage for days and then suddenly used many gigs on a single day. We recognise this is a bit annoying but at the moment there is little we can do about it, sorry.

@SaltyNZ Great thanks for clearing that up. Amazed data sessions are not cleared down at least once every 24hours minimum to cut the CDR

I just tried power cycle of handset and it looks like that will not end the data session as well (or maybe it did and CDRs just take some time to get rated)