Rbcs are within the normal range but the iron is low why?

Iron. You can have iron deficiency without having iron deficiency anemia. Anemia is a condition where your hemoglobin is below the normal concentration. And as you know there are many causes of anemia. Iron deficiency can be one of them. Iron storage ( ferritin) will be the one that goes down first before your hemoglobin declines. Why one has low iron? There are many causes-bleeding, malabsorption etc.
RBC count per se. Is not all that important. We look at hemoglobin (HGB) and packed cell volume or hematocrit (hct). If iron is low worry about low hgb, hct and small red cells (low mean cell volume or mcv).