My company is not allowed to make money on risk; we are not an insurance company; we process claims for self-insured employers; I have no incentive to "overestimate" costs; I get paid an administrative fee for what my company does - that's it. One of our principal functions each year is to estimate how much a company can expect to spend in the upcoming year on claims. (that is what actuary's for those of you following along). The actuary's are VERY good at what they do - on any group over 100 employees, they are going to be with 1 - 2% percentage points 90+ % of the time. It is amazing, frankly. AND - our actuary's are in line with the big insurance carriers actuary's most of the time. There is not "fluff" or inflation in the numbers.
Now, that doesn't mean there isn't plenty of fat in administrative costs themselves - there is. Not the least of which is the cost of negotiating and then "repricing" charges after they are incurred. I've mentioned several times on the forum that, not so much regulating prices, but making them transparent, published and uniform for all customers of a respective provider would be a good step in reducing administrative costs of claim's processing, AND allowing market forces to get some hold of costs.
What the insurance company pays is based on contractual pricing agreed to by the company and the provider - it is not arbitrary, and is not set solely by the insurance carrier. It is negotiated.