So it's not necessarily about "are you good enough?" but rather "how much do you want to get paid?" And once you focus on the latter question, you realize that skill is a tiny portion of it. Your salary is mainly dominated by how much time you have to shop yourself around, how quickly you can get several offers from different companies, and how long you can afford to say "no." The last one is very important, because if you get through an interview stage and are unable to say "no" if you need rent for example, then you'll end up with a low non-negotiable offer. You can bluff, which is always worth attempting since I'm not aware of anyone who lost out on a job offer for bluffing, but if their initial offer is $X, and you say "no, $X + $Y please" and then they say "$X + $5k is our final offer," then your options are to accept or to shop. And if you have no time left to shop around, then you're forced to settle. That's how you end up with a low salary.
All of this is obvious when someone explains it like that, but the trouble is, few people talk about stuff like this. You can spend your time from college to career without even really examining any of this, to your personal detriment, and you won't even realize it. As Patrick said, you can easily get +$5k at your next job with a minimum of effort. If you have time to shop around for several months, then you can get +$40k or more in the right circumstances.
Sorry for the ramble. These are just some unintuitive things which were surprising to me.