"A growing proportion of Medicare beneficiaries are opting out of the government-run insurance program. They are instead choosing a private plan alternative, one of the MedicareAdvantage plans. The strength of this trend defies predictions from the Congressional Budget Office, and nobody can fully explain it.
( let me add my 2 cents. if you  watched  TV lately  you would have  noticed there are a bunch of advertisements  from AARP and  insurance companies on the medicare advantage  plans . As a physician  I find it  confusing  sometimes bewildering to choose  which  health plan to choose.No wonder  senior citizens are swayed by  insurance  medicare advantage plans)

Here’s another mystery. Traditional Medicare spending growth has slowed, bucking historical trends and expectations. Though there are theories, we don’t fully know what’s causing that either.

Pinning down explanations for these two mysteries is important. Doing so could help us understand the structure and cost of Medicare in the future."