The implicit hypothesis is that if a test is full of questions you have no idea how to answer, you really ought to have strong priors that you're a below average performer. Tests are generally designed so that people familiar with the relevant material and methodologies can attempt answers; you dont need to know exactly how good other test takers are for it to be reasonable to assume you're in the bottom quartile if you can't. Same as you should have a lot less difficulty than most cyclists estimating whether your time trial was a good one relative to the rest of the field if you struggled to stay on the bike.
Of course, there are tests where the bottom quartile find the majority of it easy and have no particular reason to assume that most others found it even easier, and circumstances in which the weak undergrad who can only answer half the questions may reasonably believe that the test is being administered to a general population full of people who won't understand any of the material at all. But in general, it's reasonable to assume that if there's a lot of stuff you don't know, other people will know better.
Of course, there are tests where the bottom quartile find the majority of it easy and have no particular reason to assume that most others found it even easier, and circumstances in which the weak undergrad who can only answer half the questions may reasonably believe that the test is being administered to a general population full of people who won't understand any of the material at all. But in general, it's reasonable to assume that if there's a lot of stuff you don't know, other people will know better.