In Washington there are no private insurance companies. While all employers are required to provide workers' compensation coverage, their only choice is to buy insurance through the State or to self-insure. Insurance through the State is like buying car insurance: you pay a premium, the insurance company pays the claims, but the more claims made the higher your premium.
But self-insurers don't pay premiums, at least not exactly. Instead, they are allowed to insure themselves. This means every dollar they spend on your claim comes out of their bottom line. This means only the largest companies in the State (Boeing, Weyerhaeuser, UPS, Tree Top, etc.) are allowed to self-insure. It also means they can hire companies, known as Third Party Administrators, to run their claims. While these sound and look like insurance adjusters (and they mostly are), they work directly for your employer. If your employer tells the adjusters to authorize treatment or accept a claim, then the adjusters do exactly that.
This control comes at a cost for injured workers. Bills get paid late. Time loss benefits are not paid at the right rate. Your checks keep showing up later and later. They deny your request to go see a specialist, even though they have no legal authority to do so, and instead set you up for an IME with one of their doctors.
If you have been hurt at work with a Self-Insured Employer, you may be entitled to more benefits then what you are currently receiving. Unless someone is looking over their shoulders, these Third Party Administrators have no incentive to give you the benefit of the doubt or to double-check their work.