.
What I do not understand, it’s why some voters are so hard? Do we have to be Grandmaster painter to be lucky to have the average?!
It’s seem I’m not the only one who think that (and that it’s inefficient : even the low levels can be source of inspiration).
Indeed, on my miniatures, some puts me 2 or 3, even when they value a little more I think.
It's all relative.

Actually, there are a couple of reasons voters can be hard on a mini. Sometimes it's just someone having a bad day or being a jerk. But many times it's how your miniature happens to fall in the rotation. Pay close attention to how you vote next time. Say you're cruising along, voting, and all of a sudden one pops up that's a knock-your-socks-off, holy cow that is awesome, 10-is-not-good-enough type of mini. You happily give it a 10, admire it for a few minutes then click to go onto the next. The next one in line is by a beginner and it appears that they are either very young or still trying to figure out which end of the brush is used for painting. Because of the gorgeous mini you just saw, you automatically compare it to that one. It's human nature, we can try not to do this but it happens no matter how hard you try not to let the previous minis influence you as it happens mostly on a subconscious level. Your vote is going to be decidedly lower than what you might give if you saw a bunch of minis equal or lesser than this one.
The same thing applies if you run through a spate of less than stellar minis, especially the single photos of the same mini just different angles right in a row - and it's worse when there are 20 photos - then POW! There's a mini painted by the gods! You're going to be influenced to give it a higher score because it's relative to what came before it.
About the only time you won't have a lot of influence like this the very first mini you see. Say you jump from the forums after reading gaming posts (not other minis), or click on someone's gallery. You don't have anything in immediate memory to influence your vote - but each mini after that one will be voted on as it relates to that one to maintain your own internal judgment scale.
Basically it all comes down to, "well, I gave the last one a 5 and this one isn't quite as good as that one so I have to give it a 4 or lower."
Personally, here is how I note:
10: Grandmaster level
9: Master level
8: golden level
7: silver level
6: bronze level
5: just correct level
4: insufficient
1-3: very insufficient (but I then prefer not to note in that case).
I have a scale too but mine is...more involved. Everyone starts at a 10 and then I start deducting points for various things. -2 for obvious seams and unfilled gaps, -3 if you can actually see a hole or space. -1 for out of focus, -1 for subject too far away to see detail and so on and so forth.
Another thing to remember our opinions of our own work are going to be biased no matter how hard we try not to be. I don't mean that in a bad way as in pride goeth before the fall type thing but because only you can see the mini a certain way. You bust your buns painting a piece and just really work hard and finally when you finish it, you can honestly say This is my best work YET. Especially if you've been working on mastering a certain technique and you finally get it. Yay! Much rejoicing!
Unfortunately, no one else can see it that way. Even if you put it in the comments "this is my best mini to date" voters aren't going to get exactly what that means. I can't tell you how many times I've worked and produced a mini that blew anything else I had ever done completely out of the water - but I was lucky to get a 6 here (some of them are still in my gallery. lol!) Now, you have a right to be proud of yourself and elated that your abilities are improving - most of the time it's fits and starts - so when it comes together, celebrate. But that will be lost on most voters (unless they know you and follow your work).
Voters don't see the victories, they just see that piece at that point in time and vote in relation to how it falls in their current voting scale.