Why do I need to pay betterment for minor prior damage on a car part that has to be replaced due to a more recent accident? Wouldn't it have to be replaced anyways even if said prior damage did not exist?

Long story short, a cop ran into my front bumper+grill, and dealt enough damage for it to need to be replaced completely.

When picking up my car from the repair shop today, I noticed a betterment fee from the insurance I wasn't told about, asked about it, and the repair shop told me it was due to prior damage on the bumper.

Said prior damage was a minor dent from someone backing into me 2 years ago. The other party and their insurance refused to cover the repair costs, and since it was purely cosmetic damage, I never bothered to have it fixed.

Since that prior damage did not warrant replacing the entire bumper, I'm having a hard time understanding why I had to pay the betterment fee for this completely separate claim. The bumper had to be replaced, due to this most recent accident which left it much worse shape, so why did I have to pay for betterment? It'd be replaced anyways even if that dent was never there. Can someone please explain? thx