Very good question. It’s likely they forgot they ate the salad, there’s cross-contamination, or they had symptoms of the disease due to something else but not because of the pathogen you found on the food. We see it all the time.
It’s just like with vaccines and cardiomyopathy. We know there’s an increased risk from the vaccines, but there will be many who get diagnosed after getting vaccinated while the disease is not from it.
Just one of those things where we emphasize that correlation (and, in the case of a case-control study, association) does not mean causation.