Cells often get sold with a "# of charge/discharge cycles until you hit 80% capacity" rating, at least on the 18650 datasheets I've looked at. So my guess is that they took that number and multiplied by the range per cycle.

The problem is that (1) this is simplistic, and doesn't take into account temperature fluctuations, ac usage, etc, and (2) battery datasheets are essentially sales brochures, and often stretch the truth. (For instance, it's rare to see a battery that can actually hit the rated capacity under realistic conditions).