I know this is an old post but I think something needs to cleared up here.
From here, italics mine: http://thewirecutter.com/reviews/best-usb-battery-packs/
"We need to explain one technical detail about testing USB battery packs. As Gary Kwok, the applications engineering manager who ran our tests at Cadex, told us, the packs’ internal battery cells operate at 3.6 V, while USB runs at 5 V.
...
Battery makers, however, report the capacity of their products in milliampere-hours, a measurement of current flowing over time at a given voltage. Since we’ve changed the voltage for testing purposes, our 5-volt tests don’t offer an apples-to-apples comparison with the manufacturers’ stated capacities in mAh. The higher voltage allows less current to transfer more energy faster, so we get a lower number. But because most smartphones and tablets (and dashcams) use lithium-ion batteries with a voltage similar to our packs (and according to the laws of physics, charge is conserved, regardless of voltage), we can convert our test results back to the original 3.6 V for comparison. This step lets us directly contrast the advertised capacity in mAh (at 3.6 V) of the batteries against the tested amount of power they can deliver to a smartphone or tablet (or dashcam) operating at a similar voltage. A small amount of loss occurs in the conversion process
..."
TL;DR, don't worry about the advertised mAH being at 3.7V. It's just a number used to compare one battery against another.
