That can happen for many reasons but for one reason in particular, namely bad luck. People all have their on-days and their off-days, and they occur randomly, so doing well or doing poorly is almost purely due to chance.
Studying, prayer, and pacing, also influence the outcome but to a lesser degree.
Thats why its always important to wish people the Best of Luck immediately before an exam.
Some people get test anxiety. Some people perhaps didn't study efficiently. Some are doing bad in sections that aren't easy to study for like verbal and reading comprehension. Some aren't pacing right. Those who scored poorly probably on sections where time is a real factor probably don't know the tricks so they are wasting too much time on questions that can be solved in seconds if you know the quick way.
This is a complex question with no right or wrong answer. My own views are that those coming from more academically rigorous schools will tend to do better than their counterparts from less academically rigourous institutions. It's not to say that anyone person is smarter than another but rather one is prepared better.
I would like to believe that I came from an academically rigourous college which helped me in scoring high on the PCATs with little studying at all.
But these are my own personal views, feel free to disagree.
we're at the point to where we've been tested to death, and still have more to come after the pcat(s). you should know how you feel about standardized testing, how you do, and what you need to do to prep.
i had studied hard for months for the august pcat and got an average score, then had a sense of the exam, the pace, to bring substantive and quick to eat food for the break (it's hard to do reading comp and math while hungry) and did much better.
Personally, I feel it is due to anxiety factors as well as sleep, health, and mental ability. The better your mind is able to perform under pressure, the better one will do.
I also feel that the because I seem to always do poorly on multiple choice exams when compared to written response and short answer exams, I will expect to do worse than if this was a short answer exam. Of course, our own expectations often also have a big impact--hence the placebo effect. The information is all there and we all know it, but whether we can make the connections and recognize the answer is a whole different story.