Month to date data bill, even with a positive data balance?

<p>I checked my August month-to-date bill this morning, and noticed that there is a charge for data usage over and above my allocated data bundle, from 1 August 2017. Upon checking my data balance, I notices that I still have a positive data balance for August (over 450Mb).</p> <p> </p> <p>So naturally, my question is: how is it possible to be billed for data usage from 1 August 2017, when my data bundle is allocated at the same time? Could this be for usage from midnight on the 1st August 2017 to the exact time the data balance is allocated, in which case, how is this my fault?</p> <p> </p> <p>Please clarify.</p>