The rating of each role are based on what is expected from them. If you have a AMC that in theory should score and provide lots of assists, but instead opens up space and make many "2nd-assist", having a really good passing completion, also cutting many angels and breaking up attacks, they might not get high ratings. If they on the other hand fail to do their defensive duties and have a passing completion beneath average and instead set up goals, they will have better rating even though they wont contribute as much to the team. in the end, I would not care too much about the ratings as long as they play their part.
It would also depend on what you consider to be a poor or average rating. Personally, I woul be happy with a average rating between 6.8-7.2 for my "team players"