This is probably true for many other industries. Organisation experience / knowledge is very important in being effective in a role. When looking at productivity, organisations tend to overpay new starters (a new starter is often a net loss, particular in a skill based role) and underpay those who've been with the organisation for several years. Of course, people have to live, so you can't pay a new starter next to nothing with the promise of huge increases after 6 months.