"Scripting" is particularly hard to mark. It doesn't just include external script files and lines of code in the mission, it also includes use of triggers and waypoints. No virtue in giving credit for an external script when triggers and waypoint could have done the job just as well. Calling this category "scripting" is arguably a little misleading.
The difference between using third party scripts and ones made personally by the author is somewhere between small and zero. It makes no difference to the player who actually wrote a script. Using third party scripts well is encouraged and will give you a better mark. Chucking loads of them in in an effort to gain marks but using them badly may actually lose you marks.
Never look at any part of a review in isolation, be it the overall score or the score for one area. Always consider the whole, paying particular attention to the text of the review. Reviewing and allocating scores is difficult and reviewers aren't perfect. Neither is the scoring system itself. "Scripting" can be used - subconsciously at least - to cover "other" aspects of the mission. Overview, Briefing and Camera are all fairly clear cut, and it is important that the overall score and the score for the categories are at least vaguely consistent. For example, a mission may be worth 8 overall because of factors not covered by the categories, but Briefing and Overview are only worth 7 and camera 2. Scripting (which is the hardest section anyway) may be worth about 6 .... but the reviewer might be tempted to give 8: partly to reflect those "other" factors and partly for the sake of consistency.
Remember too that standards change over time - they are constantly getting higher. What got you a 6 a couple of years ago might only get a 4 or 5 now.
In other words, treat all scores with a pinch of salt. They are usually about right, but don't take them too seriously.