The System Usability Scale (SUS) resulted score seem to me as somewhat subjective. What ever score yields from a questionnaire for System A may just solely be just for System A, and is difficult to compare the score with System B's because of the subjectivity.
I searched around and saw this on Wikipedia:
Effectiveness in using a system for controlling a continuous industrial process would generally be measured in very different terms to, say, effectiveness in using a text editor. Thus, it can be difficult, if not impossible, to answer the question “is system A more usable than system B”, because the measures of effectiveness and efficiency may be very different.
So, it seems that this part of the article agrees that it is difficult to make a comparison between systems. However, at the later part of the same Wikipedia article, it says:
SUS has generally been seen as providing this type of high-level subjective view of usability and is thus often used in carrying out comparisons of usability between systems. Because it yields a single score on a scale of 0–100, it can be used to compare even systems that are outwardly dissimilar.
Now, it seems that comparison between systems with SUS score is possible? I am confused at this stage. Can SUS results be used for comparison between different systems for usability? That's to say we compare the SUS scores resulted from System A and System B, and conclude that either System A is more usable than System or System B is more usable than System A.