all I know is that I thought if I controlled every variable - always fed the same food, same quantity, same times of day, gave the same dose of insulin - I could expect to get the same results.
Nope. Just doesn't work that way. Insulin is a hormone and the expression in the body varies. Absorption also varies by as much as 50% from one shot to the next. It's crazy.
And then add in the "is the insulin still ok or not?" question and you can go completely insane. With Punkin, we had the added question "is the acro tumor ramping up/down production of the hormones that block the insulin from getting into his cells?"
My response was to just work with the numbers we got and try not to over-analyze the rest of it. You just can't control it well enough.
I agree with Serryn's answer. I'd try the new insulin pen, same dose, and after 2 shots from the new pen if there is no significant change then I'd assume the old insulin is still fine and I'd go back to it, finish it, then return to the new pen.