i have the sligth feeling, that you mean the number of rebounds is important for the rating and not the skill.
Even when the number of rebounds is an indicator of the rebound skill, it is also affected from random(a better rebounding team, also could get outreboundet on unlucky days) and as naker said where the rebounds happens. You got more defensiv rebounds then him, who are easier to get, which is probadly the reason, that you get more rebounds them him even when you have the lower rating.