In principle there are 3 sets of 2 wires: one for power to the unit, one set for voltage sensing (ideally directly to the battery
terminals), and one set for measuring the current
and connected to the shunt (however in fact it is measuring the very small voltage that the current
is causing). Very often and and in this case, the first and second sets are combined into one set, leaving the unit with 2 sets of 2 wires: 4 wires in total. In your case it looks like the thick black wire is used for both the amps measurement and the negative power supply.... only 3 wires by the looks of it.
I think that in your last sentence you highlighted your mistake: "treated battery as the load"
Most of these kind of panelmeters measure the current via the shunt in the negative wire. Sofar so good with the diagram supplied, however the black wire that goes from the panelmeter to the shunt is NOT the negative anymore, it will slightly higher than the negative.
Not sure how you then measure the 25.7 Volt though.
I would experiment
first with a 9 volt battery as powersupply, connected to the thin red and black wire, yellow wire to go to the positive battery terminal, and the thick black wire to the negative (battery side) of the shunt, and the thick red one to the other side of the shunt. See how you go then.
Note: connecting these panel meters the wrong way around will lead often to damage of the units, as I believe most of them have no wrong polarity protection.... I found. And if they have that kind of protection I was able to circumvent that kind protection in a very effective way