The formula for PMF is P(x;p,n) = n!/(x!(n-x)!)*(p)^x*(1-p)^(n-x), where you look at x successes in n trials, where p is the probability of success and 1-p is the probability of failure. If success is an honors and any other grade is not, then we should decide what the probability is of getting an honors. If the grading system is H/HP/P/F, then by pure chance you can assume a 0.25 chance of getting an H in any rotation and a 0.75 chance of getting anything else (again, this is chance, and studying etc have nothing to do with it). Then the probability of honoring all your rotations third year (just picking 7 to include IM, psych, FM, surgery, OB, peds, and neuro) is 7!/(7!(7-7)!)*(0.25)^7*(1-0.25)^(7-7) = 1/16384 = 6.10e-5, while the probability of honoring none of those rotations is 7!/(0!(7-0)!)*(0.25)^0*(1-0.25)^(7-0) = 0.133. So, you have a much lower chance of honoring all your rotations, but your chances of not honoring a single one is not very good either.
So the population of students who honored all or none purely by chance will be pretty small, though the people who don't honor anything will be more numerous.
But yes, my point actually is that while I'm sure luck has something to do with it, there is definitely more going on. Having 5% of your class honor every rotation means it's more than luck.