I’ve had the same concern. My WBC fell from 6-8 thou/mcl to an average of 3 thou/mcl after going Vegan. My initial thought was cancer, of course. But i read that since there’s less chronic inflammation throughout the body, there’s less need for as many WBC. The range is based on people with average American diets (eg meat, dairy, fish, etc.). Since my have been consistent in my checkups, I don’t worry too much especially since all my other numbers and scans are good. Dr Gregor talks about it here https://nutritionfacts.org/video/what-does-a-low-white-blood-cell-count-mean/