Obviously that's silly, but there is certainly SOME subjectivity.
But when it comes to comparing across makes and models, we need not worry about that, unless there is a consistent tendency in reporting...
For example, would you expect Toyota customers to over-report reliability compared with, say, Ford owners?
If reliability is a '9' for Toyota and a '7' for Ford (just making up numbers of course), could you explain away the difference due to 'subjectivity'? If so, what would your argument be that Toyota owners are over-reporting the reliability of their cars?