Percentiles and Spread
Where you sit in the lineup, and how wide the lineup is
A percentile is just your position in a lineup. Scoring in the 80th percentile means you did better than about 80 percent of the group. It says nothing about how many questions you got right.
The 90th percentile is not a score of 90 percent. On a brutal exam you could answer 55 percent correctly and still land in the 90th percentile, because almost everyone else did worse.
The shape of the crowd
Pile all the values up and you get a distribution. Some look like a bell, with most people near the middle. Some are skewed, with a long tail of big values. Some have two humps, meaning two distinct groups where almost nobody is actually average.
Spread matters as much as the middle. Two cities can both average 60 degrees: one hovers between 55 and 65 all year, the other swings from 20 to 100. Same average, completely different wardrobe. Statisticians measure this with range and standard deviation, which is roughly the typical distance from the middle.
Whenever someone hands you an average, quietly ask: how spread out is it? An average without spread is half a story.