make sure the thing has been under load for an hour then pull out your multimeter. you will be surprised.
Of course it will change values you add load, thats just a basic concept of electric circuits. Adding load=more resistance which will directly effect the current and voltage. In fact, the moment you connect a multimeter to you circuit, you're adding load (there is a resistor inside that thing). This partially accounts for readings being "off" by a bit no matter what you're reading. That and the fact that no components are actually ideal values when you actually use them.
If you're getting readings off an adapter that are realky off (not even remotely close to what they should be, then you have a defective charger (either that or your meter is on the fritz .
Ohms law equation: V=IR
V=voltage
I=current (engineers are weird

)
R=resistance
That equation (like any equation) can be rearranged to find any of those values based on the values you have already.
Ohms law ALWAYS applies in dc circuits, once you get into some of the fancier AC components thats not always the case.
That being said the psu converts ac to dc while stepping down the voltage. All the adapter outputs is dc meaning Ohms law applies no matter what as far as the output goes.
This is a basic principle of electricity and physics. Unless something is drastically wrong with your adapter, the output (ideally) should not change. Of course due to the fact that nothibg is ideal in practice/reality there will be minor discrepancies and fluctuations but it will always stay within a reasonable ballpark.
Not trying to be rude here, just pointing something out. If you're getting major discrepancies on your adapters output, either the adapter is bad, or your meter is .