that doesn't make sense! if it's claimed to be 8Ah then it has to be, likewise, it should charge your crapplephone 6.5 times (actually nearer 6 times), either the big pack isn't getting a full charge, or the current limiting for the Iphone is done with some cheap resistor circuit thing, wasting most of the charge current as heat, does the phone get hot when charging? or the charger?
failing that, run the Duracell pack as low as you can then give it a full initialising charge (ignore any "pack full" or "charge complete" lights), give it the full eight hours or whatever they say it needs for first charge, that might cure it, with many high capacity packs you MUST give them the full initial charge in order to reset the charge/discharge controller and switch it from the factory "pack storage" mode to "charge cycle" mode, if you don't do that the pack manager gets confused and misreports the main pack state, shutting it down to prevent over-discharge when it still has plenty of juice left because it thinks it's almost empty due to not being reset properly when first charged.
edit @Gruso
yeah! thats true, but 50% efficiency is way out, he should be able to squeeze 5 charges out of it imo, when running and cycling flight packs I find the chargers report a close correlation between what they put in and the capacity of the cell (better than 5%), my chargers are pretty fancy programmable things mind you, they may be more efficient than some freebie supplied with the pack/phone
edit:edit: hah!, probably sussed it!, for a guess the pack is a single cell 8000mah LiPo, that gets pumped up to 5 volts to charge USB devices, the extra voltage = reduced capacity (ie double volts=1/2 capacity, 3.2 volts > 5v = capacity divided by 1.5625, 8000/1.5625=5120mah (at 5v assuming 100% efficient conversion), that's 4.19 charges, if you assume the converters in the phone and the charger/pack eat up the differential, not being 100% efficient, as you say, you can't really get a charge when the pack is discharged to the same level as the Phone, so subtract 0.6/0.7 = 3.5 charges, so 3.5 recharges sounds about right (imo), if he could charge the Iphone cell directly off the Duracell cell then he would get his 5 charges, the voltage pumping/dropping between the pack and the Iphones electronics is eating the difference (I think), pumping up to 5v and then dropping it back to 3.2 is losing a whole chunk of power.
