I recently purchased the prefab countertops you find in the big box stores.

After spending 8 hours of ripping out old countertop and sink and prepping new the tops for install, I find the miter joints aren't even close to flush. I stupidly assumed they would fit since they were pre-cut.

The corner joint at the front and the backsplash come together but leave a huge gap in the middle.

I've tried some sanding at what appear to be high areas but am afraid I may have made things worse.

Buying 2 new pieces will hurt, but its not out of the question.

Is this common with these prefab pieces? What should I be doing to remedy these bad pre-cuts ?