I downloaded 40GB on the last evening of my month. The data between 11pm and 12 got carried into the next month.
The summary usage metering system was wrong, but the detailed usage system knew that it was putting the data into the wrong month, because the data was absent from the list.
Then when the bill comes in, the system has the temerity to admit that it's charging me for data from the last month. You can see that the invoice period starts from the 5th, but the bill includes the 4th.
Another way of looking at it, is that the billing period starts from 11pm on the previous day, for no obvious reason.
I think this will exist on anyone's bill, if they downloaded in the last hour of the month.
If you want the usage stats in your modem to line up with the Optus summary, you have to avoid downloading in that hour.