I know this is all very anecdotal, but I certainly have seen developers that I wonder for months how they could have ever been hired, and given time become quite capable. I first started to notice with out of college developers, but eventually saw beyond that too.

In my experience, the ones that were 'capable' from the get go either had a lot of experience programming on their own, or had some internship or other real world work experience. The ones less capable didn't.

Then I realized that my measure of 'capable' was probably unfair, because I expected people without having worked before to be aware of all the complex details that working with other developers entail. All the annoyances you have to deal with about writing real code that needs to evolve and change that only a stable work environment provides.

Then I took it one step further. What if the ones that weren't out of college were just never expected to really improve. They never had the chance to be exposes to a stable work environment that required growth. They just either were let go from previous jobs before being allowed to grow, or weren't put in an environment that neither required nor promoted growing? I think not all jobs do a good job of demanding and providing space to grow. And that's a problem. It's possible to 'get good' on your own, but it shouldn't be expected; and it's far from the only way.

So maybe we are being a bit unfair when we say a candidate is weak. Maybe they just haven't been in an environment that would require and allow them to improve.