How in-network rates differ between networks & administrators?

Ultimately a good thing for me, just super curious. I recently switched jobs/companies, moving from Texas to Ohio. Both my old (>100,000 employees) and new (5-10,000 employees) have self-funded group plans. My current employer’s plan is administrated by UMR (Choice Plus Network), while my old employer was BCBS TX (Blue Choice PPO), though employees in some states had UMR Choice Plus.

Anyways, I get these injections every 3 months, and the “allowed amount”/in-network rate in Ohio (UMR) is materially less than I was paying in TX (BCBS). The injection dosage and procedure are exactly the same, at (I think) similarly-sized, well-known academic medical centers. The TX facility billed for more services, but, just looking at the drugs injected (brand-name, same quantity), TX submitted $14,265, while OH submitted $10,000. The BCBS in-network rate was $7,600, while OH was $4,600.

So, my question is, why is there such a big difference for the exact same thing? Wouldn’t insurance companies want to stay competitive with each other? Or is there some function of location and facility in determining the rates? In other words, since TX billed more and has generally higher costs of living, they get more? Alternatively, if location doesn’t matter, would my old employer have saved several thousand each year by putting me on UMR? I hit the out-of-pocket max either way, but this lower cost will have me spreading that out more vs. paying most of it at the beginning of the year.

My new employer also pays more of the costs ($0 employee premium, 15% vs. 20% coinsurance, and $3,000 vs. $4,000 annual employee max), so I’m not sure if that has anything to do with setting the in-network rates. No complaints here, just a bit intriguing.