If this is a deferred annuity and you received a non-periodic distribution, taxable gains come out before any of the nontaxable investment in the contract. Only if you are receiving periodic payments as a result of having annuitized the contract is a distribution a mix of taxable gains and nontaxable investment in the contract.
If you report a different taxable amount than the amount in box 2a and do not provide a suitable explanation, expect a letter from the IRS claiming that you underreported this income.
