While political gamesmanship in a modern bureaucracy may certainly influence the extent of the differences; the primary cause is simply the vast difference in individual aptitudes--always in the context of the demand for those aptitudes at any given moment in social history--and personality traits which affect the extent to which one develops one's individual aptitudes.
Nothing is more ridiculous than the collectivist/egalitarian mantra, that tries to demagogue this reality.
William Flax
Difference in individual aptitudes is where the hard work comes into play it does pay off in the end.