I noticed the same: on flank formations, the MC's are more focused on defense than offense, as the passes go down the flanks. Generally, people boost attack more than defense in the midfield. Personally I've had more success with MC ratings when passing was mixed or through the middle. Still, ratings dont always make sense to me. My GK occasionally scores 5 or 6 when I had 0 goals against me. Perhaps its a simple matter of having less to do that makes ratings lower. Less chances to 'shine'.