Dental Insurance Question/Clarification

I started a new job last year, and since then, I've had a lot of issues with my dentist, and I'm hoping someone can help.

Essentially, my dentist has my insurance info. They calculate what I should owe based on coverage, etc and I pay it at the time of service. But the last 2 times I've gone, I've gotten another bill like a month or so later. It was a big run around last time, and I ended up getting my dentist office and insurance on the phone. And basically the insurance company said something to the effect of "we only cover a up to a certain amount per service in a given region".

It seems they are saying that even if my policy states they cover 75% of a tooth extraction (just as an example) they can say "but in Chicago, we will only pay $500 regardless".

So is this a valid thing that happens with insurance? Or is my dentist scamming me? Or are they just wildly out of sync and overcharging compared to other places in the area.

I want to trust the dentist, because I never had these issues in the past. But its getting to be ridiculous, so not sure if I maybe should find a new dentist.