Some variability could just be due to chance. For example if someone definitely knows 15 of the questions, and has to guess the other 10 with a 50% chance of being correct for each, then they have at least a 5% chance of getting 2 of those 10 (hence 17/25) or worse, and they have at least a 5% chance of getting 8 of those 10 (hence 23/25) or better.