I judged mine based on craft beers. For instance, I would give Widmer hefe a C-C+ and so if my beer was on par with that then I would rate it the same. Most people who try my beers would rate them B+ to A- but they aren't as adept at picking out flaws. My biggest concern is having UBBs (ugly baby beers) where only I love them. So I constantly look for good feedback, always asking people "what DON'T you like about it?" I think people are just being honest when they say C/B because they have tasted grade A beers before.
I hear you. And somewhat agree. For instance, my last couple of pale ales honestly rival some craft brews. But to give myself an A, they should rival the best craft brews. I'm working on it. I think that to improve, we all need to identify and correct any flaws in our brews, even if very small.