Back at the screen where you chose to enter your drug information, you also had the chance to go directly to the plans. The Medicare.gov site gives an overview of Prescription Drug Plans, Medicare Health Plans (Medicare Advantage Plans), and Medicare Special Needs Plans (SNPs). Based on your "Health Status" questions (Health as Excellent, Very Good, Good, etc.), Medicare is able to estimate the amount of your prescription drug spending and listing Part D plans that are most affordable for someone in a particular health class.
Another interesting feature of this view is that the Part D plans are show with their Drug Cost Sharing. The standard Medicare Part D prescription drug plan provides a 25% cost sharing in the form of co-insurance. In this listing however, even plans with co-payment models are represented showing the range of possible co-payments.
This is simply an estimation of your costs. It can deviate significantly from your actual experience.