To me, the Grade A vs Grade B here was in the prep and planning. When you are trying to match something, you really need to measure every detail. It's not as easy as blaming it on grade, etc. When you have the material in hand, grab a tape measure or ruler. It would seem to me that a book edge lamination (basically cut and folded so all the grain matches) could have been cut to any depth so it would match the existing. If they merely stacked the pieces and assumed they would match depth, then even the variance between 20 and 22 mm in your Grade A case could be a 4 mm difference. The difference between 17 and 22 could be up to 10 cm (about 1/3 inch) when doubled. But you are already have the oops -- now how to deal with it.

Do the two surfaces join? If not, can you step back and see if it is really that big of an issue? The 4 mm is a little over a 1/8 inch and most folks wont notice that if you aren't pointing it out unless the meet at some point. If it is that kind of thing, then I might try living with it a while and seeing if my eyes didn't stop focusing on it after a it was brand new.

If the two surfaces join, then it would seem your two options are to have the laminated edge shortened to match the other (a mess - and needs to be done carefully or things could get worse, so make sure you trust your fabricator and their approach for doing it) or replace the other half and make sure it is done right.