According to [1] a 1 meter USB cable can have a resistance of 286 milliohms, for a voltage drop of 0.8v over a meter (taking the resistance of four contacts and two lengths of cable into account).

So if you have a '3A 5V' power supply without a captive cable, you can still get 4.1v at the useful end of the USB cable.

The official RPi chargers include a captive cable, presumably to stop users substituting such standard USB cables.

[1] https://www.westfloridacomponents.com/blog/compensate-cable-...