Dehydration: This is the most common cause of an elevated red blood cell (RBC) count with a low mean corpuscular volume (MCV), as dehydration concentrates the blood, artificially elevating the RBC count while the MCV remains low due to the underlying cause of the anemia or normal cell size.

Chronic Disease: Certain chronic diseases can lead to a normocytic or microcytic anemia (low MCV) and, if there's an associated increase in RBC production or a decrease in plasma volume, can result in an elevated RBC count.
Smoking: Smoking can increase RBC production, leading to an elevated RBC count, and if the smoker has iron deficiency anemia or another cause of microcytosis, the MCV would be low.
High Altitude: Living at high altitudes can stimulate erythropoiesis, increasing RBC production to compensate for lower oxygen levels, potentially resulting in an elevated RBC count. If there's an underlying cause for low MCV, such as iron deficiency, this could also be present.

Polycythemia Vera: A myeloproliferative disorder that leads to an overproduction of RBCs, white blood cells, and platelets. Although typically associated with a high MCV due to the increased cell size, early stages or specific variants could present differently, making it crucial not to miss this potentially lethal diagnosis.
Renal Cell Carcinoma: Some renal cell carcinomas can produce erythropoietin, leading to an increase in RBC production. If there's a concomitant reason for a low MCV, this could be an underlying cause.

Erythropoietin-Producing Tumors: Similar to renal cell carcinoma, other rare tumors can produce erythropoietin, leading to increased RBC production. These are less common than renal cell carcinoma but should be considered in the differential diagnosis.
Congenital Disorders: Certain congenital disorders affecting erythropoiesis or red blood cell morphology could potentially present with these findings, though they are exceedingly rare and usually identified earlier in life.