Here's something that came to mind.

I don't know which thread it was, but Waterlogged commented on how usually you plate a copper layer first, to assure good adhesion, then you plate the nickel.

If this is done, then the rest of the block could theoretically be a lower grade copper, which is then sealed off by the high grade copper plate that attaches the nickel to the base copper.

So here's a possibility - EK's plater isn't doing the copper layer first, assuming the copper is high grade already? Maybe because EK said it is (because maybe it is supposed to be and they're getting batches of cheap stuff thrown in the mix?)?

Just ideas coming to mind.