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I'd say there is something special in the cord to auto set the amperage pulled from the host.
USB ports can vary the amount of power pulled from the host port, with the USB spec showing anywhere from 100 to 500 milliamp (if I remember right, may even be just a step from 100 to 500, and nothing in between). However, sometimes fishy things can be done to step it up, or adjust it. They may trade off voltage for more amperage, or even just a voltage step down. USB is 5 volt, and for smaller electronics, 3.3 volts is a popular figure. If this voltage stepdown is done via a resistor in the cable (which may very well be the case, to save space in the watch), that would explain the differences.
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