I'm sure that somebody will be able to describe what's happeneing more clearly but I'll give you my take on the situation. You are probably making insulin, maybe not as much as you need. Your cells are probably also insulin resistant - so when you eat, you release insulin which acts on your cells to accept the glucose in your blood for energy. But either there isn't enough insulin to activate the receptors, or the receptors don't recognize the insulin as well as they used to. Your cells take in as much glucose as they can- and the rest stays in your blood (that's why you stop at 250).
If you don't eat, your liver releases glucose from the glycogen that's stored there- It won't let your brain go without glucose- even if it has to raid its stores, and the stores in your muscles- so it makes glucose for you if you fail to give it what it needs- and then the same cycle as before kicks in with the insulin rise, insulin resistance and so forth.
As for how your pills act- different types of meds act in different way- some cause your pancreas to make more insulin, some stop your liver from making so much glucose, some extend the life of certain hormones, some slow down the rate at which your stomach releases food into the small intestine- all these actions cause the glucose level in your blood to go down, or become more stable.