That's likely because USB (as strictly defined by the spec) can only put out a limited amount of power. Some devices are made to be able to handle more than is

That's likely because USB (as strictly defined by the spec) can only put out a limited amount of power. Some devices are made to be able to handle more than is strictly necessary to meet USB specs. So given a comparatively powerful AC outlet as opposed to a USB power source your iPhone can charge more quickly.