It's Dimorphic Anemia
You are right it should affect the same red blood cell.
Celiac disease produces anemia by affecting the absorption of nutrients needed for the build up or red blood cells. In most cases you have Iron and folate deficiency (remember that Iron and folate are absorbed in the jejunum which is the most common part affected by celiac disease) but if the disease also affects the ileum you may also get B12 deficiency.
The most common observation (statistically speaking) is Iron deficieny anemia (microcytic hypochromic). However, if both Iron and folate are deficient then you'll get what's called the "Dimorphic Anemia" in which the RBCs are hypochromic (less Iron) and at the same time macrocytic (arrested development due to folate deficiency). The biggest hint to dimorphic anemia is the high RDW which means there's high degree of anisocytosis (different sizes of the RBCs).
In USMLE they usually wants to you know that the anemia can be either Iron or folate deficiency anemia and so they may throw at you a case in which you see both CBC elements but in real hematologic practice this is seldomly seen you usually see dimorphic anemia.