The purpose of the Contribution Column on the Categories & Items page is to give the instructor a breakdown of individual items within either the natural weighting or the adjusted weighting rolled up as the entire course. It corresponds to the User Report that gives the student a breakdown of how their final grade is being computed. This is especially important when an instructor adjusts the weights, which make adding up points meaningless.

That said, there should be a contribution total for the course on both the Categories and Items page and the User Report so the math is transparent.