Successes are easier when you're trying to impede rather than try to overcome an obstacle, thus the wider success range for impeding (1-3) over overcoming an obstacle (5-6). Successes in overcoming grant you points, while successes in impeding take away another player's points. The classes just help give each player advantages and disadvantages depending on the obstacle at play - an obstacle matching your highest stat being better this round over an obstacle matching your lower stats. Essentially, every class gets one obstacle where they shine, one where they are poor at what they do, and one in the middle. Two characters could also impede one character in an action, so a character overcoming an obstacle they're good at might be impeded by the two other characters.