Elevated RDW

I had two blood tests about a month and a half ago and was told by the doctor that my counts are "consistent with iron-deficiency anemia". I was told to take iron supplements for a month, then have another blood test (and to continue taking iron for two months after that). I have been taking iron supplements for a little over a month and just got the results from the latest blood test. My hemoglobin, hemotocrit (this was always okay), and MCV are now within the normal range, while my MCH, MCHC are still low but have gotten better. My platelets were also high, but now are almost within normal range. S-TfR also went from being too high to now being fine.

Only two counts got worset, one of which is my RDW (the other is my WBC, which went from 11 and 10.9, to 13.1 - reference range 3.4-8.2). My RDW went from 15% and 14.9% to 17.2% (reference range >15%). What could cause this increase in RDW?