Extra nail biting

My female cat is a nail biter. For the 10 years we had her, she has always had short, frayed nails on her front paws only. She bites them short constantly and we never knew why. Her back nails are fine though.

She has plenty of stimulation in our house (toys, whole house to roam, windows to look out, and sometimes she goes outside in our yard supervised). She used to be a feral before we adopted her, so we understood the transition we would need to give her lots of inside stimulation to prevent boredom.

She has plenty of scratching posts in the house she uses.

The vets had her on fish oil for a year thinking she was lacking something that made her bite her nails or make them brittle. The fish oil made her coat shinier and gave her some minor gas, but changed nothing with her nail biting behavior.

She seems fine otherwise, her nails are never bitten to the blood vessel so no bleeding, and if she scratches you the nails still are sharp enough to make you bleed and she scratches her scratching post fine, she can jump and climb just fine so it doesnt affect her mobility. We never had to clip her nails.

What causes this behavior?

I couldnt get a picture but I saw the nails. Some of her nails are fine, others have that husk around new nails growing, but it appears the new nails are bitten at the tips so they dont curve they are straight. Her normal looking nails are curved, but are very tiny, or at least tiny in comparison to our male cat who has massive claws.

Funny though, our male cat has no extra husks around his claws they are long and clean, we do see them come off whole once in a while from him, but my female cat's husks are all frayed and short when they come off. If she didnt bite them I think her nails would be fine, I just dont know why she bites them so much, her back claws are normal she leaves those alone.