You have some weird ass categories OP, inventory? I mean i guess I know what you're talking about but is it worthy of its own category, probably not. Where's the graphics category, the presentation category? I put N/A for difficulty because you can choose it, plus there's accessories that make it even easier if you decide to use them, plus i don't know what constitutes a high score for difficulty? does 10 mean ultra difficult or just the right amount of difficulty.
Inventory entails the system that shows all your items, how they're kept or organized, maybe if the game is extremely detail oriented you get descriptions of each item, where to find it, other valuable information such as hints or suggestions, recipes, etc. Interface matters for a lot as well because who wants to look at an ugly menu showcasing what you have? People want an easy on the eyes, simple, yet pleasing interface that will display everything with ease. Take the game EarthBound for example. It has an excellent inventory system for its time. Fun to look at, fun to buy, the items were interesting, items were also valuable in many situations and especially for survival, you have your younger sister and others iirc who would store your items if needed. Inventory is a big part of RPGs so why wouldn't the category be considered? Like if I asked you how many hours did you spend looking at and considering your inventory when preparing for battles, healing during battles, preparing for advancement past dangerous areas and the adventure ahead, considering how best to adapt and strengthen your character(s) it would probably be a few hours of time that added up right?
There are other significant categories worth rating too, I agree with you there. Categories such as graphics or I guess artstyle same thing almost, and then there's lore, sound effects, pacing, characters, controls, etc.
If a game receives a ten out of ten in difficulty, I would consider that game challenging right out the gate and it remains challenging, thoroughly so, throughout the entire course of the game. Could be the perfect amount of difficulty or difficulty that causes the player to die plenty of times, rage even after failure, get stuck at certain spots, all these things. I prefer for games to be very challenging. Even if the mechanics are somewhat janky and cause frustration that's fine by me. Better than being easy and mindless imo. It's all subjective