With the discovery of the MAGI for the purpose of a traditional IRA deduction having the same problem, my guess is that there is a bug in the calculation for the MAGI for the purpose of a TIRA deduction and then this badly calculated MAGI is modified to come up with the MAGI for the purpose of a Roth IRA contribution instead of doing an independent calculation of the MAGI for the purpose of a Roth IRA contribution.
