My blood tests have come back with high (out of range) RBC and low MCH and MCHC. What could be the cause of this?

Thal minor? You did not give specific numbers but this is common is thalassemia minor, with red cells too small being produced in generous quantities to make up for their smallish size. It's worth knowing about because marrying someone else with it can produce children with thal major which is very bad.

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What can cause my blood tests to come back abnormal for my rbc?

RBC. The red blood count is a concentration and reflects the balance between red cell production and destruction. Normal RBCs survive about 120 days, but that may be shortened if they are defective, coated with antibody or damaged by drugs. Deficiencies that impair production of hemoglobin, the key ingredient in rbcs, the bone marrow may respond to make more cells. Other stimuli can also alter the rate of production, including neoplasms and inflammation or kidney disease. Read more...