That is interesting, I wonder if they are reacting to the fierce sun, drought and heat, which is not a brassica favourite, especially those varieties which are best in spring and early summer. For example, Greyhound was superb in the cold rain of last June.
So yes it is varietal and you would be better sowing say Marner Early Red Cabbage (in MArch) for July and August harvests, if you want that. I just picked a lovely dense heart of it.