Iga was measured to be low at 40 mg/dl. range was above 68mg/dl. dr said it isn't iga deficiency because it's too high. what caused this then?

No IgA deficiency: Your IgA is a little lower than the average. It is not low enough to be called deficiency. Iga deficiency used to be IgA of less than 30mg/dl, occurring in 1 in 500 people. A level of less than 1mg/dl occurs in 1 in 2000 people. The primary cause is unknown, but there is failure of certain b lymphocytes to mature into IgA producing plasma cells. If you have no symptoms - no treatment necessary.