My fully charged HyperJuice external battery does not last longer than what is advertised.

The advertised result is based on the hours of wireless productivity obtained from a fully charged HyperJuice and MacBook.

When a fully charged HyperJuice external battery is plugged into your MacBook, it was designed to recharge the internal battery and provide juice to the MacBook at the same time.

So, with the Macbook having low battery, an internal battery will draw more energy from HyperJuice to recharge itself, as a result the hours of power supplied will not last longer.