Capability to earn

I have a question about the definition of underearning – which Brooke defines as: you’re earning less that you want to – you want to earn more and you’re capable of earning more. But, isn’t our capability defined by our thoughts? E.g. “She’s capable of X” is a thought, not a circumstance, correct? If that’s true, can’t any of us be capable of anything we truly desire? The way underearning is defined makes it sound like capability is fixed. I believe you can grow in your capability and I feel like that’s what life coaching is all about – helping people step into their true potential, which they might not even be able to see themselves. Please help me demystify this definition. Thanks!