The dimorphic red blood cell (RBC) population flag is caused by an abnormal distribution curve of red blood cells due to more than one population of red blood cells in the sample. The mean corpuscular volume (MCV) parameter doesn't correspond to real size of the red blood cells in those cases.
The red cell distribution width (RDW) parameter is unreliable too or it is not delivered by the analyzer at all. The dimorphic red blood cell population flag can be raised by samples with extremely high counts of lymphocytes or macrothrombocytes too.
But it is caused by an interference with an electrical impedance method for measurement of RBC in those cases, not by more populations of RBCs.