Your premise would be correct if you were dealing with a medication like an antibiotic. Insulin is a hormone. It's action is influenced by any number of factor such as your cat's activity level, stress (or lack thereof) since stress hormones are steroids which cause an increase in BG number, time of day since there is diurnal variation in stress hormones, your cat's weight, food (and sometimes the manufacturers change the carb levels and don't tell us), variation in a cat's routine, other cats, etc.
In other words, you may be doing exactly the same thing day in and day out and your cat's numbers are widely different. It a cat or a squirrel wanders into your yard, it may cause a rise in stress hormones and Max's numbers go up. If he eats less, his numbers may go down.
Like Crista said, some of the variation you're seeing may be a result of bouncing. If Max's body has gotten used to higher numbers, any time numbers drop low, drop fast, or simply drop into a range that's lower than what he's used to, a stored form of glucose along with counterregulatory hormones (i.e., steroids) are released which cause numbers to rise. It can take 3 days for a bounce to clear.
