I keep wondering how accurate is that. On one hand implementing that precisely surely must be difficult - if it wasn't, everyone would have it. On the other hand, Azure clearly have issues calculating the bills in real time. Heck, once I kicked off a bunch of large VMs for a day. Once shut down I checked out the billing page - the costs estimates kept increasing every hour!

My hypothesis is that they don't really have it nailed down but given big margins they have they can afford to let you use more resources than you pay for in the end.