IRS changed refund

Have done taxes with Turbo Tax for several years.  2024 taxes filed this year was supposed to get refund of $2011.  Received letter from IRS saying they changed the amount of taxable Social Security benefits on our tax return due to an error in the computation of the taxable amount.  They referenced publication 915, Social Security & Equivalent Railroad Retirement Benefits for more information.  We received a refund of $511.  Did Turbo Tax not compute this correctly?  I just used the amounts from the 1099's that I received...