dmtaylor above answered your question exactly as I would have answered, I think I've seen reports he mentions on this very forum.
Regarding the difference between the yeasts, you aren't kidding yourself, they are slightly different indeed. That's true for each and every yeast Mangrove Jack source from Lallemand. Their matching pairs (Nottingham/M42, Köln/M54, Windsor/M15, London/M10, Abbey/M47 etc.) are recognisably similar yet slightly different flavourwise, probably because MJ do not just repack but also blend their yeasts. To my liking, MJ's versions always taste a bit better.
Maybe they really do know what they are doing, making custom blends. Maybe none of them are pure equivalents, but only partial. I myself ain't pulling out a microscope to know for sure.