Additional Medicare tax - why is turbo ax recalculating additional medicare tax Already paid through employer and included in my W2 ?

It depends. One possible reason is that you are married. Additional Medicare tax is often computed on your tax return when a married couple files jointly and their income exceeds $250,000 together. The reason that it is computed on your tax return and not through payroll is that one person only needs to pay the additional tax once their income exceeds $200,000.

Here is some information you may find helpful: What Is Form 8959: Additional Medicare Tax