Plans are not static. A drug that sat on tier two this year can move to tier three next year, a medication can drop off the formulary entirely, and premiums shift. The plan that was the cheapest option for you last October may not be this October, and nothing tells you that automatically. The only way to know is to re-run your list.
Bring your current Medicare card, a list of every medication with its dose, and your current plan information. The comparison takes about twenty minutes and shows total annual cost, not just the premium, which is where most of the difference actually shows up. Whether you switch or stay is entirely your decision.