Speaking for my reviews, and I'm sure most of the MG reviews at that, the overall score is usually not an average score of the separate parts: it's a combination of each score plus an overall 'tilt' value, an addition (or subtraction) of points that reflects the overall package of the game.
Also, we have to factor in more than just the hard numbers e.g. the 'graphics' score usually reflects the technical quality of the visuals, but points might be subtracted from the overall score if, for example, the art direction is horrendous, there are clipping issues, etc.
For me, the final score is how I feel about the game overall; as Tarryn says, the whole is greater than the sum of its parts.