TurboTax does the correct calculation. Since a SEP contribution is an employer contribution, a SEP contribution effectively reduces your compensation on which the maximum contribution is calculated, so a special calculation must be done. A 25% base rate results in a maximum SEP contribution of 20% of net earnings from self-employment being the maximum contribution by a self-employed individual.
See IRS Pub 560, Chapter 5: https://www.irs.gov/pub/irs-pdf/p560.pdf
