I think the inside tags should say white or unbleached but then it's been a long time since I had new(ish) regular cotton. The few cottons I have left the tag is long gone.
That is odd though, I would expect that your older ones would be lighter. Unbleached diapers definetly get a lot whiter over time, eventually becoming just as white (or pretty darn close) as bleached diapers. I guess the batch thing is probably the only explanation. I know if they'd sent you white by accident you'd probably know right away because they're really gleaming pure white. Definetly odd though...