So, my follow up question is, as a printer, would you supply a client with two jobs each using a different paper supplier, when the quotes specified the same cover specs and they would be delivered together?! Seems mad, one feels totally different to the other, and they really shouldn't.
Did you ask the printer for an explanation as to the difference?
The trick is to get the jobs printed in the same place and chances are the printer will use the same stock on the same press, so you get a bit of continuity.
Is the printer based in the UK - did the box containing the print arrive from, say: Germany?
Well it might just be that they had some separate stock that needed using up and they thought you would never notice - can happen.