The uneven distribution of contributions doesn't matter so much on Github, since when an employer goes to scout out someone on Github, they're typically looking at the code itself, rather than the quantity of it.
A good metric for measuring ability of git hub user is to look for followers of his repos. Github is useful only when someone is making large amount of contribution or significant contribution, such as solving a hard problem.
A college can give someone A- in introductory algorithms and that gives you some confidence about his skills in algorithms. However github cannot provide this sort of credential, with same amount of confidence. Github can spot "exceptional" programmers, since large or significant contributions are more difficult to fabricate.