PT ‘amount billed’ doubled!?

I’m sorry if this isn’t the point of this subreddit, and would be happy to post elsewhere but I’m not sure where.

So I had an injury not too long back, and was finally well enough to start PT in May. I went for a couple sessions just to see what it was like, and the practitioners/staff are all nice enough, but they work for a relatively large hospital chain (Memorial Hermann) here in Houston. Anyways, each session, I had to pay out of pocket as I was working towards hitting my deductible, and per the claim statement at the end of the month, it looks like MH was charging the insurance company ~$600 per session (split into therapy/exercise at $300/$300, generally.) Anyways, I only paid ~$150 per session because of insurance plan discount, so I was fine to keep going.

So I went a couple times in June, and around then I also hit my deductible, so insurance starts to pay 80%. But I recently got a bill from MH and I went back to check the claim MH had sent my insurance, and instead of $600 per session (with the $300/300 split) they were now charging my insurance company $1200 per session – $600 for therapy and $600 for exercises!! Keep in mind, my treatment had stayed *exactly* the same, same staffers, exercises, everything. Again, since I have the 80% covered by insurance, what I paid stayed nearly the same, but this feels like an extremely deceptive business practice from Memorial Hermann?

Is that actually legal? I’m pretty shocked that an hour of stretching/body weight exercise could be charged at this rate, but I was ok with the $600/hr since it was being reduced to $150 for me via insurance… But now I feel like Memorial Hermann is taking the piss, since they know through checking my insurance that I’ve met my deductible, and are just charging more for the exact same? What’s going on here?