Observing and exploring the relationship between current and potential with a USB multimeter
This phenomenon is voltage drop due to internal resistance of the power supply and its wiring.

Somewhere in the laptop is a voltage regulator. It will adjust the current supplied until its output is at 5V. This is connected both to the internals of the laptop and the USB ports.

However, between the power supply and the voltage measurement point is some wiring, which appears to have a resistance of about 0.4 ohms.