Before adding the $3600, your itemized deductions (mortgage interest, real estate taxes, state taxes or sales taxes, charity, etc) must have been under the standard deduction. When your itemized deductions are under the standard, Turbo Tax keeps the standard deduction in place. A small amount of the $3600 must have gotten you over the standard hump. Now that you are officially itemizing, an addition of $50 saves you about 25% in taxes.
A $1000 savings from your donations would only be logical if your other itemized deductions already exceeded the standard deduction. In your case, it is clear that you needed a large chunk of your itemized charitable donations just to meet the standard amount.
Standards:
$6300 Single, Married Separate
$12,600 Married Joint
$7600 Head of Household
plus additional allowance for blind and/or 65+ years of age
