If it’s an off the shelf kitchen, carcasses are generally not perfect in size or even squareness.
That is why you have such a great amount of adjustment in the hinges and drawers.
A good fitter can make a silk purse out of a pigs ear.
They are generally poorly mass produced chipboard boxes and the bling is in the doors, drawer fronts, cornices, end panels and fillets.
A truely bespoke kitchen normally comes finished but the fitter may have to shave a little here or a little there, and then they send some one to reinstate a perfect paint finish.