The differential count is a breakout of the numbers and percentages of the different kinds of white blood cells or other cells found in your blood, and would refer to things like MONO, LEUK, LYMP, EOS and BAS. These counts are usually included with a thorough blood workup but aren't always ordered.
Do you have any results for MCV (mean corpuscular (red cell) volume), MCHC, PLAT or RDW? They would help to characterize any potential anemia or other cytopenia they might be concerned about.
I'm not a doctor but think the numbers of your WBC (white blood cells) and RBC (red blood cells) are low. It would be interesting to know whether your platelets (PLAT) are low too. These findings can be spurious or could be caused by any number of conditions and I'm sure the first thing the hematologist would do would be to run a full set of tests covering a variety of measures including rerunning those the results of which you already have. If you have low numbers of white and red cells then it is possible that any potential problems might not be limited to iron deficiency issues that would respond to iron pills, and hence the referral to the hematologist for a further workup.
It is easy to say, but don't let all of this worry you, just go to the hematologist, take along a detailed list of any questions you have and be sure to make sure you get answers that you understand. They probably wouldn't decide for sure on whether to do an aspiration or biopsy until they do a month or two of bloodwork to see what the course of things may be. I have been through a similar process and have a few increasing abnormalities in my bloodwork. My red cells are too few and too big and getting bigger and fewer, and my platelets are too small and dropping in number (but the number is normal for now). I will probably have a marrow aspiration and biopsy done by the end of the year (but am trying to enjoy the state of denial for the moment).
In my case, we think we have eliminated just about any condition from the differential diagnosis except myelodysplastic syndrome, which is a diagnosis that can only be made based on biopsy results. It is something they would probably consider in your case, as based on the results you've mentioned, you have low counts in at least two of the three blood cell lines (white cells, red cells and platelets), but you may have normalized or had a response to some kind of treatment that would eliminate it as a concern by now.
Best of luck, and just try to stay as informed as possible. Get a second opinion if you have any question about anything they tell you.