Anyone I've seen advertise themselves as an expert in something (as opposed to being named or termed an expert by someone who's hired them to work/speak etc) seems to be a generalist with ego or confidence issues.

Developing expert-level skills in an area requires focus, talent, and time. These three variables are all present but vary in concentration in 'expert' folks I've met operating within various disciplines and niche sectors. Some are initially more talented, so their 'expertise' development takes less time. Some have less native talent so spend more time developing skills.

Unless you're a prodigy, the development of expert level skills requires persistence most of all, but persistence is not necessarily equivalent to time spent practicing.

To barnaby's point, everyone I consider an 'expert' doesn't feel like they're done learning - they remain voracious.