Ammeter show half of the real current draw
These meters put the current measuring shunt in the negative supply line.
care must be taken to avoid all ground loops.

Also note they are average reading, not RMS, so should only be used with reasonably stable sources.

Also the thin black wire terminal connects internally to one end of the current shunt (and from the photos it appears most-likely to connect to the red end)  if you connect it to the wrong end it will parallel the shunt causing low current readings.

I would re-test with an passive load like an automotive brake lamp to be sure that the load negative terminal is isolated from the supply negative.

The thin black wire should only be used if you're using an isolated supply to run the instrument, if you're connecting thin red to thin yellow to power the meter from the voltmeter input, or any other supply not isolated from the current being measured) leave the thin black unconnected: cut it off, remove it from the plug, or insulate it.