Some schools use (or once used) formulas that add points to GPAs from certain schools. Boalt Hall once used such formulas (I don't believe they do any longer) and made them public. Williams GPAs got a decent boost in those rankings. However, the truth is, school prestige won't help much in the numbers part of the game nowadays. An LSDAS GPA from school A is equivalent to an identical LSDAS GPA from school B no matter what the reputation of the schools. When index scores are calculated, it doesn't matter where the GPA came from.
Williams does give you an advantage in one regard. Once in committee, I think the school name sometimes buys a little extra respect. This is in part because the average student at Williams has an LSAT score in the mid 160s. So, to some degree, a middling class rank and GPA are not as damaging as they are at other schools. If you can get the right LSAT score, even a "low" GPA at Williams will get you into some great schools.
You go to a great school. You've got great opportunities. Don't sweat your GPA too much. Work hard, do well on the LSAT and you will have your pick of schools.