This question will probably be easy for someone to answer. Let's say I'm in the 32% tax bracket, but not right at the upper limit. As an experiment, if I add $100 of interest income to my tax return, TurboTax reduces my Federal refund by $36. If I add $1000 of extra interest income, TurboTax reduces my refund by $358. I was expecting the reductions to only be $32 and $320, respectively. What could account for the difference? Thanks.
