Since it does show contact only at the corners, I'd suspect it does have something to do with being unconstrained now that it is sawed out. If this is a sacrificial piece, grind back the corners a bit and see if the contact spot grows, or disappears. I'm sure it could be lapped flat, but as thin as it is I wouldn't count on it being stable.
I watch the installers put in Italian marble on the nurse stations at the hospital, and it comes in pretty good size slabs, all pretty much cut to print. Where the piece has to be longer than they can practically handle, they shim and play with it until it is very close, and epoxy it all up, then polish to blend. It takes a long time, but the seams are invisible. There is some talent involved. You cannot even see a flaw by eye from a low angle. They have it down really well. But marble isn't as hard as granite. I wonder about the abrasive they use. I'll have to ask or get some next time we do a remodel.