Reducing our taxable income to below 80,250 from 22 to 12% changed refund incrementally and not by 10%. Why not?

1) Depends on how much you reduced the income by, as the 22% is only applied to the income above a certain level.

2)  If you have self-employment income...then it depends on what kind of income you changed.  The SE tax only changes when your net profit changes.