The above graph is for Focal FPS 4160, as you can see in the graph, the distortion varies with the output and power applied. See THD is different at 1 Watt whereas different at 100 Watts. It varies with power applied and also keep in view that this specific graph is measure by a signal of 1KHZ. Will at be same a t 2Khz? NO.
So the crux is, not all the manufacturers specify the exact numbers. So unless you don't know how it was measured and at what instance of frequency or power, then it will be misleading.
Also THD and THD+N are two different things. While measuring some manufacturers take sum of 4 HD's while some take 6 HD's. And Noise (N) can be obtained by eliminating a source (reference) signal from the measuremnt.
Same theory goes with the S/N ratio.
Rest @DrShaggy can elaborate it a bit.
Edit: Also the time interval "t" for the measurement (Length of measurement in time domain) changes the numbers as well