For federal annuity 1099R why does Turbotax not reduce the taxable income line 2a by the health insurance premium amount line 5?

The payments you make through your annuity for health insurance are not deducted from your taxable pension amount. 

It is the same as when you were active service, the employer pays part and you pay part. 

The amount on line 5 is your portion. 

You can enter the amount paid for health insurance as a medical expense, which would be deductible on Schedule A if you Itemize (with restrictions),(Additionally, if you take the Standard Deduction, medical expenses are not used as a deduction) but your payment for insurance does not reduce your retirement income. 

Thanks ... this is my first year retired and I would have thought medical insurance would be deducted from retirement income as with regular income.