Regardless of whether the committees meet every week or two, it's not comparable to law school admissions. The difference is there is no such thing as a "borderline" candidate at a callback. Everybody is very well qualified, and the firms are going to offer the people they like. There are no externalities at work like yield protection or numbers that affect rankings. The people who don't get offered from a callback are people who aren't going to get offered no matter when they interview.
While I agree with LonghornDub's remarks, I have to say that I agree with others about having a slight advantage by being considered for employment earlier in the recruiting season.
If there are 100 summer associate positions available, and you interview when 0 offers have been made, it is much easier from a mental/psychological perspective for the hiring committee to offer you a position than if you interview when 98 offers have already been made.
Firms offer callback interviews to candidates in which they are genuinely interested. However, as with all positions, there are more qualified candidates than available positions. Your qualifications will be considered with respect to other candidates.
Additionally, firms budget for how many summer associates and future first-year associates they expect to have. At some firms, the legal recruiters are given guidelines as to how many screening interviews and callback interviews to offer. While firms can obviously hire more summer associates than they initially intended, it is much easier to be hired for a budgeted position than for one outside the budget.