Maybe we have to concentrate on the BONUS of MG (Yellow triangle), no ranged unit has one (except for the bonus vs cities, but we can assume that MG doesn't have it).
Maybe is a bonus against artillery? but MG already have "Cover 2" by default.
And if MG have a special bonus like "shot at first sight" like the Citadel? (the map building created by Great Generals, im not sure about the name, my game is in italian)... maybe i'm travelling too much with my mind
Cover 2 by default is confirmed? Where?