You're not bragging, but perhaps you're a match for the culture? Your communication skills are better than your colleagues? You collaborate better across teams, and get more done? You're better at planning, and delegating?

I'm trying to say that you being promoted twice and not being an excellent engineer doesn't mean it's easy. You may have other traits that allow you to excel in that environment. The level distribution by itself says that being promoted twice in 3-4 years, with a significant bump in pay each time, isn't the norm.