Cost of living, this and every year after I'd been working for 10 years. Adjust the salary vs years of experience data to dollars of constant value and you'll see that it amounts to 10% of starting salary per year for the first 10 years, followed by little to no net salary growth thereafter.
But that's just salary. I also get a bonus that you wouldn't believe if I were at liberty to tell you...Let's just say that I've had an upper decile salary for 9/10 of the past decade. And I richly deserve it, given how much money I personally make for my employer.
Have I ever told you how much I love my job?! And I'm not just talking about the pay...
My employer believes in shared pain for shared gain. Base salary sucks, but the bonus system is very, very generous- if the company remains profitable. It's in everyone's (direct, financial) interest to keep the customers satisfied AND the operation profitable...amazing the corporate harmony, cooperation, innovation etc. that this generates.
Some really dumb new folks negotiate a higher starting salary in return for less bonus, leaving more for the rest of us. But no shared pain equals no share of the gain- they get theirs up front every month, whether we're profitable or not. They don't take more than a year or so to change their tune generally. Even dumb folks smarten up eventually!
If we were a public company we'd never get away with paying ourselves so well- when the business is profitable. Fortunately the employees are also the share-holders, and the non-employee share-holders know which side their bread is buttered on and don't mess with the system.
We do some oil and gas business, but we also do business for the people that use oil and gas to make products. Their business has completely dried up, but the oil/gas folks are spending like drunken sailors and we're happy to take our share. During the long run-up in oil prices, the oil consumers' business dried up quicker than the oil- producers spending grew. Fortunately we kept everybody- and everybody took it in the teeth on the salary- or else we'd have been totally unprepared to sieze this market boom.
The Dilbert guy has it nailed: my favorite strip has the pointy-haired boss saying, "It's our policy to hire only the best technical professionals". Dilbert replies, "And is not also our policy to pay the industry AVERAGE?". The boss replies, "That's the way we like 'em: bright, but clueless"...