It's Verizon rounding up and a reminder to refill or pay. They do this with data too. It isn't just lag in reporting, it's just the way Vzw does this. A lot of customers don't watch their usage and just go with the alerts as a measuring tool, and Verizon knows it.
It always intimates that I will go over if I don't do something. Seems to me if they would just tell the truth they would do themselves better.