chance mutations, recessive genes, Caldari salivary glands and/or pancreas produce one or more extra enzymes, which happens to neutralise the toxic component of Kresh leaves, turns it into some other substance that is non-toxic.
if you go with salivary glands producing enzyme A, that turns Kresh toxin into substance X (less toxic), and pancreas producing enzyme B, that turns substance X into entirely harmless molecules, as well as enzyme C that turns Kresh toxin directly into harmless molecules, then it means that full-Caldari people have all 6 genes to fully digest Kresh, and can do so at a high concentration, because of the simultaneous action of the A+B enzymes and the C enzyme. Still possible to overload the reaction with a sufficient concentration of Kresh toxin.
A half-Caldari person might be lucky, and their non-caldari parent has a gene that allows producing of either enzyme A or C, making the Kresh drinkable. If they only have enzyme A, then they might be ill, but not severely. They couldn't drink it at a high concentration, because they only have one of the necessary enzyme reactions.
Make the mutation that allows the salivary gland to produce enzyme A fairly common, and the mutations that allow the pancreas to produce B and C less common, then most of the original Caldari Prime population could drink the Kresh at a low concentration, because of being able to produce A, with a few lucky people being able to drink it with no effect, because they produce B or C as well.