Overpaid Social Security tax is when a single taxpayer has 2 or more employers and those employers together withheld more than $7347 in Social Security tax (W2, Box 3 and 4). Any amount above $7347 from 2 or more employers is refunded on Form 1040, line 71.
If you have a figure on Line 71, and it doesn't seem right, ensure that you didn't list your spouse's W2 as your own (or vice versa). This can cause that refund to incorrectly generate.
