My medical expenses were 17,570.95,my 3% of income was 696 Thus I was expecting an alloowed medical expense of 16,875 . Why was it 12741?

Since your 3% was $696, your Net Income must be in the region of $23000.

Hence the sum off your Personal credit and allotted Medical credit needs only to equal this amount in order to reduce your tax payable to zero. The remainder of your Medicalcredit was not used., and is non-refundable.

You might be able to claim less credit on your 2017 return, and save an amount for a possible 2018 claim.. You cannot however claim for the same expense twice.