Varathane Help!!!!

I need some help! I am refinishing an art deco era vanity for a customer. The desktop of the vanity I stained with Rustoleum Kona stain and I had so far sealed it with 3 coats of Varathane Crystal Clear Satin. My plan was 5 coats of Varathane. The desk top was beautiful and I was showing my husband how nice it looked when I dragged my fingernail on it and it scratched!!! This was probably 3 full days after applying the 3rd coat of sealer. I don't mean I was digging at it with a screwdriver, I mean I lightly ran my nails back and forth...normal wear and tear type test. We tested a few other areas and it scratched every time. I wasn't concerned since I had to lightly sand before my next coat of poly, but when I started sanding, the scratches wouldn't sand out. Before I realized I had sanded down to the wood and the scratches are still visible!!! That means my scratch test went through 3 layers of poly and into the wood. My entire vanity top now has to be sanded to bare wood and redone. Is this the way Varathane is???? This is the first I have used it. It was sent to me by Rustoleum because I had issues with a different sealer. The whole point of all those layers of poly is to protect the wood. I should not be able to scratch it!!!! Any input would be greatly appreciated.