I can't go into too much depth as to how FIFA rates their players, but I can tell you that there is a general feeling among sim players that the ratings under the hood are way too high and it clearly effects game play. Too often you'll see players from lower division teams with low overall ratings pull off moves and passes that should be reserved for the likes of Messi, Ronaldo, or a Neymar.
For all of the great presentation and graphics, FIFA for years has suffered from cheap, lazy, old school programming when it comes to overall gameplay and the player ratings are part of it. Would love to see a roster editor for FIFA!
The comments in this thread seem to confirm the idea that all of EA's sports games use a rating system that is outdated and nothing more than a piece of the hype machine. I even remember using re-rated rosters for NBA Live that made a tremendous impact on game play. Live enabled you to globally edit the specific ratings!