What happens when you do the same experiment with the inline
DC amp meter?
What happens when you reverse the direction of one of the wires
so that the currents subtract?
Is it really DC current?
If the current is not really constant, the meter has an algorithm
that converts what it sees to a DC current value for display.
Average? RMS? other? The waveform of the AC components of the currents
will affect
the converted number that the meter calls DC depending on what
algorithm the meter implements and the shapes of the two waveforms.
You also have to worry about saturation in the meter core.
Your loads are likely pulse-width modulated and have a peak current
well in excess of the average...maybe...
If the peak current exceeds the range of the meter, the top end
will get compressed.
Put the biggest caps you can find on the load end of the printer
connections.
Measure the current in the wires between the caps and the PS.
If the caps are BIG, the current will look more like DC
and the readings should converge toward what you expect.
Doesn't look like it here, but watch out for paths that
circumvent your measurement. Ground here is not the same
as ground over there. It's possible to have current go in directions
that don't include the wire you're enclosing.