My answer is very much related to the question asked here:
very-basic-insulin-using-question
So, @kpanda01, please join us here!
I want to use very simple numbers to make it easy to illustrate. And we will just use the food and insulin as perfectly balanced to make it easy to show the principle.
So this is fantasy land, where all the numbers line up, okay? (Just like your endo or CDE thinks it works. 
)
Let’s imagine your IC (or ICR or whatever the medtronics call it) is 1:15.
Suppose you are 85 and you want to eat 30 grams.
Simple math says that is 2 units. It would be perfect, right? Exactly 2 units for 30 grams.
And let’s say your insulin duration is 4 hours.
So yes, 2 units will make your 30 gram meal go away perfectly. You started at 85, and the 2 units and 30 grams will perfectly cancel each other out, right?
But when? When do you get back to 85?