This is expected and described in the product support information by Apple. The MagSafe Battery Pack has a 1,460mAh capacity at 7.62V. This translates to roughly 2,920mAh since the iPhone has a 3 ...

For more information read our Complete guide to Apple MagSafe: What is MagSafe ... While it can’t match the higher capacity 10K battery packs for recharge power, it is lighter and slimmer ...