Blood counts usually my rbc cells are always or most of time a fixed count every test but my wbc differential are never a fixed count why .thanks?

Reference interval: There is such a thing as intraindividual variation where your reference intervals of analyte vaues can vary within 2 standard deviations which can vary throughout the day, week or month. The WBC count would be especially sensitive and would be affected by the invading agents/exposure around you or just your circadian factors or the rise & fall of your stress hormones would do it.