Interesting research shows that the degree of blood sugar control achieved in type 1 diabetics is determined in large measure by genetics. The study showed that glucose control achieved by diabetics was closely correlated with the glucose levels in their non-diabetic, pre-diabetic relatives (who were still in normal blood sugar range), which suggests that glucose control is heavily influenced by genetic factors independently of self-discipline, patient training, frequency of monitoring, diet, exercise, etc. Some bodies, it seems, are just naturally designed in such a way that the blood sugar is less stable.
Source: H. Sneider, et al, "HbA1c levels are genetically determined even in type 1 diabetes," DIABETES, vol. 50, no. 1, p. 2858 (2001).