You get unusual percentages when you start factoring in advantage and disadvantage, Veronica - in the example, you might have a 13-in-20 chance of rolling 8 or over an a d20 which is a 65% chance, but if you have advantage, the 'expected probability of getting 8 or above on at least one die, of the two, jumps up to 87%; the pool of possible rolls has increased from 20, to 2p20 (possible permutations of 2 results from a bracket of 20 numbers), which is 400, while the number of successful combinations is (13 x 20) + (7 x 13), or 351 possible combinations of two die that would be a success; 351/400 is ~87% (88 if the game rounds normally).
It's not intuitive, though, when you think about rolling dice... and abstracting it to that extent can be mentally misleading...
