Confusion: Power Bank and Mobile Charging
The power bank is rated for 10.4Ah. To get the WattHours of the device out you must multiply the capacity of the battery by its nominal voltage (not 5V, the output voltage). For a lithium cell, this is usually 3.7V. 

3.7V*10.4Ah = 38.5Wh for bank capacity. 

To charge an iPhone, you need 5V, the device is only 93% efficient turning the battery's 3.7V into 5V so now the energy capacity is 38.5Wh*.93 = 35.8Wh. 

35.8Wh/5.92Wh = 6, still a discrepancy.

The iPhone internal circuitry now has to take that 5V input and convert it to its desired nominal battery voltage which is probably around 3.7V. There will be more conversion losses here, and it is impossible to say exactly how much. Even if we knew the exact efficiency of the iPhone circuitry the numbers still wouldn't work out perfectly as explained below.

A battery's capacity is never a concrete number, and varies with temperature, discharge rate and many other external effects. Maybe the 10.4Ah is only true if the load is 500mA or less. It was likely determined by the manufacturer that a fully charged bank could only charge that model phone 4.5 times through testing or simulation of their battery makeup.