I don't follow your logic here. Typically ranged units have first strikes, exactly because of that. They managed to shot a round at their enemy even before they got close. Similarly to the Drill promotion line, which mainly enhances that advantage. Where did you get this info? Hills have +25% defence, Forests +50% (exactly to be a huge disadvantage for mounted units). At the same time there is no penalty for open terrain which mounted units "ignore", their superiority there is represented indirectly.