It's not so much that Bob is bad at math. Rather, he has no mental framework to handle numbers beyond the ones he deals with every day. Bob sort of knows that a billion is more than a million and a trillion is more than a billion, because he learned it in 8th grade. But he has no idea of how much more. As far as Bob is concerned, all those ' -illion' words are interchangeable and mean 'a really big number, like ten thousand or something'.
Bob's problem isn't a lack of knowledge; it's a lack of any appreciation of scale. You can correct him, you can explain the difference between a million and a trillion countless times, and it won't matter. Bob thinks you're just being pedantic. He's mentally filing you with people who correct him on 'your' vs. 'you're'.
Bob's not a particularly religious person; he doesn't go in for young-Earth creationism. He knows the world is ten million years old, or ten trillion years old, or something like that. He even heard somewhere that cavemen and dinosaurs didn't actually live at the same time. But his mental image of Dinosaur Times still includes woolly mammoths and saber-toothed tigers as members of the ecosystem. Okay, a more scientifically-minded person might get all pedantic and nitpick that picture, but Bob's no paleontologist, okay? He has his own life to live, his own worries to worry about.
A couple of years ago, a story circulated about a survey that asked Americans whether the government was spending too much, too little, or just the right amount on foreign aid. Definitely too much, said Americans. How much should the government be spending, the survey asked. About five percent, said Americans. The punch line is that the actual amount the government spends on foreign aid is far, far less than five percent. This survey was generally spun to mean that Americans know far too little about how their government allocates money.
I wouldn't argue with that, except that Bob and like-minded people formed a substantial portion of the survey respondents. Bob doesn't know the difference between 5%, and 0.5%, and 0.05%. Well, okay, on one level he knows the difference, because he managed to pass eighth-grade math all those long years ago.
But he doesn't really know the difference. He wasn't having a flashback to eighth-grade math when he took the survey. He was talking about government. Not math. They're different things. He was reaching for an expression to mean 'a small amount of', and he came up with 'five percent'. Now you're going to tell him he picked the wrong expression? You nitpicky pedantic twit.
I don't know if Bob's lack of familiarity with scale is innate, or if better math education at a critical point (long before college) could have helped him.
But years of reading Internet comments from a wide swath of humanity has convinced me that Bob represents a fairly sizable proportion of the population. They're out there, and simply correcting them isn't going to solve the underlying issue.