Food intake, although a large component of blood glucose variability, is only one factor affecting your glycemic control. Exercise or physical activity is another. However, even when changes in physical activity and medications aren’t a part of the equation, there are many other factors that play into making your blood glucose a moving target.
One of these is the insulins we have available. NPH insulin is notorious for having large intra-individual variability (meaning you can get widely different results each time you use it), but even the newer insulins, Lantus® and Levemir®, are not anywhere near 100 percent reproducible every time. One study by Heise et al published in Diabetes found that the serum concentrations of insulin Lantus® and insulin Levemir® varied by 24 percent and 18 percent when participants received four same- quantity doses of the medication.
Other reasons you do not get reproducible blood glucose numbers are that the glucose output from your liver is not consistent every day and that your starting blood glucose readings will have an effect on the timing and action of your insulin.
Even your emotions and stress can play havoc with your numbers. Essentially the human body is not like a test tube: all variables can’t be controlled for
So, expect that you are never going to be able to reproduce the perfect lunch numbers you had today even if you choose to have a repeat of your meal tomorrow.