This is probably the thread that @stevehoyt is referring to:
But if the there is a discrepancy between your BOTH your devices tab AND the bubbles, VS the Total Usage number, I would suspect that something different is going on. When you see these discrepancies, is there an Other bubble or Other total in the devices sections ? If not, you could be dealing with a situation where your calculated Always On value is temporarily too big, leading to a negative Other that displays essentially as zero. In this case, your bubbles and devices will add up greater than your total usage.