Why did insurance only pay for a cheaper treatment?

A tooth-colored filling on a back tooth is often paid as if it were a silver (amalgam) filling. Using example fees from the American Dental Association:

• Your dentist's in-network fee for the tooth-colored filling: $90
• The plan's fee for a silver filling: $60
• The plan pays 80% of $60, which is $48
• You pay the remaining $12 of the silver-filling fee, plus the $30 difference between the two fees, for a total of $42

• Crowns on back teeth: paying for a metal crown instead of porcelain.
• Replacing missing teeth: paying for a partial denture instead of a bridge or implant.
• Deep cleanings: if the plan's review finds that fewer teeth qualify than were treated, it may pay as if a smaller deep cleaning was done. The reason comes from the plan's criteria; see how plans decide on deep cleanings.
• Tooth removal, including wisdom teeth: removing a tooth is billed at different levels, from a simple removal up to a tooth fully stuck in the jawbone. If the plan's review of your X-rays finds the tooth was less stuck than the level billed, it may pay for a simpler level. The reason comes from the plan's criteria; see how plans decide on tooth removal.

Your Explanation of Benefits may say the cheaper treatment “could have been performed.” That's a payment rule, not a judgment that you got the wrong treatment. The two options often have real differences in looks, durability, or how much tooth is removed. Ask your dentist to explain them.