I don't know about the Ping. I use the Deltec Cozmo. I use a Dexcom, CGM, too. Pumps calculate IOB in various ways, but the basic idea is this. You have programmed (or the pump default is) say, that a bolus lasts 3-4 hours in your bloodstream. That 3-4 hours is the period of time when you have IOB, meaning that you already have insulin chugging away in your system based on a bolus you gave some time ago.
I don't know your numbers, but let's say you bolused 12 units and that your "duration" factor is 3 hours and that your pump calculation is a rather "straight line." So, at the moment you bolus, you have 12 units IOB. One hour later, you have 8 units IOB (because 1/3 of the insulin is used up); two hours after the bolus you'd have 4 units IOB; and finally, at 3 hours, IOB = 0.
Pumpus use this feature to remind us mere mortals that we DID bolus a while ago and that insulin is still working. Hope this helps a bit.
BTW, I'd love to have a flatline graph on my Dexcom. It doesn't happen often or for long, though ;-)