It's just a really well-known phenomenon. People thinking they don't stink because it's the smell they're used to. I'm just speculating, and there's of course a chance I'm wrong.

As for how I know I stink, i have a much keener nose than the average. I can tell when my hair stinks because it's been too long since my last shampoo, for instance, a thing most people can't even tell about others apparently. Even though it's the grossest smell ever.

But even then, most of the time I can't tell my breath stinks. It probably has to do with how close the mouth is to the nose. Mostly, I've associated sensations in my mouth with instances where I was told I stunk and work from there.

As for what I eat, oh yeah, I absolutely do eat carbs a normal amount. But I really don't think this should have much of an effect when I'm not eating. I have trouble imagining bacteria having a stock of food for when I don't eat.