"I suspect that the hospitals have some agreement with the insurance to get that discount, or they are doing this only to foreigners in Thailand, or the cashier made a mistake this morning and should have deducted the insurance's bill."

You're suspicions was right @gmichelbkk.. most of the hospitals here in Bangkok had their agreements done including the doctors. I'm a Filipino living here in Thailand about 10 years now and got married with a Thai, @thidaratapple, for 7 years. I let my wife read your content about what I'd just copy paste and she said that if they (hospitals) see some foreigners with insurance, they will just make a story or something to get all of the money in their (foreigners including me) insurance. In addition of from my wife, Thai people, too, are being pushed to get the money in their insurance.
As of my experience before, I had bitten by a centipede at my back. It's kind of big actually, and my co teacher took me at a private hospital. I got 100,000 worth of insurance and they did what they have to do. (I think you know what I mean hahaha), They checked me up and did what the nurses must do and when I looked up from my receipt, hell yeah, 6,560 baht for just a bit of that silly centipede.

Well, I hope to meet you soon here @gmichelbkk somewhere around. 😊