When income tax is raised, some people work less or shift income to lower-taxed forms, so revenue grows less than the static estimate. A cut works the other way.
Disputed assumption – may not work this way in reality
Economists disagree whether this effect works, and how strongly.
There is broad agreement that behaviour responds, but not on how much: estimates of the elasticity of taxable income range from about 0.1 to over 0.5. With a small elasticity the effect is smaller than this, with a large one clearly bigger.
Of every 100 euros that a change to “All earned and capital income taxes” saves or brings in, 25 € is lost as revenue on the line “Personal income tax” every year. (The other way round if the change adds spending or cuts taxes.)
Share of the change's own effect on the balance, year by year while the change continues: −15% means 15% of a saving comes back as cost (or 15% of extra revenue is lost). With the default values.
You can switch the effect on or off and adjust its assumptions in the simulator under Cause-and-effect chains.