I believe there is a mistake somewhere regarding your twin siblings' premium for Medicare.

Not necessarily. It's kind of complicated. The basic premium for Part B was $96.40 in 2009, $110.50 in 2010, and is $115.40 in 2011.

And there's a twist. There have been no SS COLAs for two years, and there's a provision that Medicare premiums for SS recipients can't increase by more than the SS COLA - so if your Medicare premium is paid from your SS benefit, it's frozen at the current rate. If you don't pay via SS, you get the hike. So while my wife and I were born just a month apart, and started on Part B at pretty much the same time, I'm paying $110.50 (the 2010 rate) and she's now at the 2011 rate of $115.40. That's because my premium is paid through my SS (and thus is frozen at last year's level), while my wife gets the new, higher rate (she doesn't have SS, and pays her premium through withholding from an annuity).

So my guess is that the twins signed up for Part B in late 2009, getting the 2009 rate of $96.40. And their premiums are paid through their SS benefit, and so remain frozen at $96.40. On the other hand, if you signed up in early 2010, you'd be at $110.50. (There's yet another twist relating to high-income individuals, but that doesn't seem to apply here.)

Lorenzo