Even in humans - some are more susceptible to vitamin and mineral deficiencies than others. If they also had access to pasture and greens it would reduce the effect. Also, if the corn/maize was treated it can make the niacin it contains biologically available for the animal. Corn and maize do contain significant amounts of niacin it is just not bio-available unless it is treated.
Agreed about being an individual thing. Interesting about treatment. Corn/maize has a reasonable amount of niacin in it. How do they treat it? One way of getting more out of it is fermentaion or sprouting, I imagine.
It gets cooked with an alkali - usually lime. I don't know if fermenting would have the same effect.