Electronic – Measuring current with multimeter

It's because of the circuit used inside of the multimeter to create a virtual ground at approximately Vbat - 6.2V, or about 2.8V for a 9V battery; this basically means the COM terminal on the meter is at 2.8V relative to battery ground and readings will be offset by this.

I tried with another meter to measure its 9V battery - it read 6.08V, which is very close to the predicted 2.8V drop.

Measuring the cheap meter on my desk, the 10A range uses a 0.1 ohm shunt resistor and the 400 mA range uses a 2.5 ohm shunt resistor and the 4mA range uses a 100 ohm shunt resistor.

100 mA through 0.1 ohms is 10 mV drop and 100 mA though 2.5 ohms is 250 mV drop. So depending on the impedance of your circuit, the current could be lower in the lower range just because of the increased series resistance.

You can measure the voltage drop across the ammeter with another voltmeter.

When you can't tolerate voltage drop, try a feedback ammeter: http://www.keithley.com/data?asset=6169