Part D Companies/Costs

First, I'd like to thank everyone in this subreddit. The information that I have received since the beginnning of my research as I transition from an employer policy has been invaluable.

I'm looking at Part D drug plans. I take only two run of the mill drugs on a daily basis and might even have one dropped in a couple of months. Both of my drugs are covered under the medicare formulary. Here's the question (I'm going to make up the numbers):

Why would Plan A be 2.30/month with a $590 deductible, Plan B be 48.18 with a $590 deductible or Plan C be 35.15 with a $0 deductible?

This doesn't make sense since they all cover the same medications. What am I not seeing?