Ohm's Law: V=IR, or R=V/I
The implication is that R is a constant for a given object. Turns out to be not neccessarily true for all objects.
So we redefine Ohm's Law: r=dV/dI
r is now a variable, and is calculable. Ohm's Law is now not a law, just a definition of resistance, which can change with circumstances.
Power supplies: for simplicity, we assume it can be modelled as a constant voltage source with a built-in (constant) source impedance.
Then measuring its open-circuit (no load) voltage gives us one parameter, and measuring its short-circuit current gives us the other.
But if we have something - a length of naff cable for example - between the PSU and our meters, we are going to get a different (wrong?) answer. Having another meter gives us the opportunity to characterise that naff cable as well.
I buy a PSU complete with cable, and it says "5V 1A", I expect to get that out of the end of the cable. If I find I can't, I can look into it. Maybe it's just the naff cable, maybe the PSU before the cable, or maybe both. Interesting, either way, but at the end of the day, not useful - I can't ask the supplier for just a new cable or just a new PSU, he'll just replace them as one item.
More to the point, even if that PSU had done what the tin said - give out 1A @ 5V - by the look of it it could roast my Pi, kill me, or burn my house down. Or someone else. Not only should we keep well away from these things, we should make a song and dance to get them taken off the market.
Like nuke China, perhaps